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<div class="document" id="iterator-concept-and-adapter-issues">
<h1 class="title">Iterator concept and adapter issues</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Date:</th>
<td>2004-01-21</td></tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr>
</tbody>
</table>
<div class="contents topic" id="index">
<p class="topic-title"><a name="index">Index</a></p>
<ul class="simple">
<li><a class="reference" href="#issues-from-matt-s-tr-issues-list" id="id1" name="id1">Issues from Matt's TR issues list</a><ul>
<li><a class="reference" href="#iterator-access-overspecified" id="id2" name="id2">9.1 iterator_access overspecified?</a></li>
<li><a class="reference" href="#operators-of-iterator-facade-overspecified" id="id3" name="id3">9.2 operators of iterator_facade overspecified</a></li>
<li><a class="reference" href="#enable-if-interoperable-needs-standardese" id="id4" name="id4">9.3 enable_if_interoperable needs standardese</a></li>
<li><a class="reference" href="#enable-if-convertible-unspecified-conflicts-with-requires" id="id5" name="id5">9.4 enable_if_convertible unspecified, conflicts with requires</a></li>
<li><a class="reference" href="#iterator-adaptor-has-an-extraneous-bool-at-the-start-of-the-template-definition" id="id6" name="id6">9.5 iterator_adaptor has an extraneous 'bool' at the start of the template definition</a></li>
<li><a class="reference" href="#name-of-private-member-shouldn-t-be-normative" id="id7" name="id7">9.6 Name of private member shouldn't be normative</a></li>
<li><a class="reference" href="#iterator-adaptor-operations-specifications-are-a-bit-inconsistent" id="id8" name="id8">9.7 iterator_adaptor operations specifications are a bit inconsistent</a></li>
<li><a class="reference" href="#specialized-adaptors-text-should-be-normative" id="id9" name="id9">9.8 Specialized adaptors text should be normative</a></li>
<li><a class="reference" href="#reverse-iterator-text-is-too-informal" id="id10" name="id10">9.9 Reverse_iterator text is too informal</a></li>
<li><a class="reference" href="#prior-is-undefined" id="id11" name="id11">9.10 'prior' is undefined</a></li>
<li><a class="reference" href="#in-other-words-is-bad-wording" id="id12" name="id12">9.11 "In other words" is bad wording</a></li>
<li><a class="reference" href="#transform-iterator-shouldn-t-mandate-private-member" id="id13" name="id13">9.12 Transform_iterator shouldn't mandate private member</a></li>
<li><a class="reference" href="#unclear-description-of-counting-iterator" id="id14" name="id14">9.13 Unclear description of counting iterator</a></li>
<li><a class="reference" href="#counting-iterator-s-difference-type" id="id15" name="id15">9.14 Counting_iterator's difference type</a></li>
<li><a class="reference" href="#how-to-detect-lvalueness" id="id16" name="id16">9.15 How to detect lvalueness?</a></li>
<li><a class="reference" href="#is-writable-iterator-returns-false-positives" id="id17" name="id17">9.16 is_writable_iterator returns false positives</a></li>
<li><a class="reference" href="#is-swappable-iterator-returns-false-positives" id="id18" name="id18">9.17 is_swappable_iterator returns false positives</a></li>
<li><a class="reference" href="#are-is-readable-is-writable-and-is-swappable-useful" id="id19" name="id19">9.18 Are is_readable, is_writable, and is_swappable useful?</a></li>
<li><a class="reference" href="#non-uniformity-of-the-lvalue-iterator-bit" id="id20" name="id20">9.19 Non-Uniformity of the "lvalue_iterator Bit"</a></li>
<li><a class="reference" href="#traversal-concepts-and-tags" id="id21" name="id21">9.20 Traversal Concepts and Tags</a></li>
<li><a class="reference" href="#iterator-facade-derived-template-argument-underspecified" id="id22" name="id22">9.21 iterator_facade Derived template argument underspecified</a></li>
<li><a class="reference" href="#return-type-of-iterator-difference-for-iterator-facade" id="id23" name="id23">9.22 return type of Iterator difference for iterator facade</a></li>
<li><a class="reference" href="#iterator-facade-minor-wording-issue" id="id24" name="id24">9.23 Iterator_facade: minor wording Issue</a></li>
<li><a class="reference" href="#use-of-undefined-name-in-iterator-facade-table" id="id25" name="id25">9.24 Use of undefined name in iterator_facade table</a></li>
<li><a class="reference" href="#iterator-facade-wrong-return-type" id="id26" name="id26">9.25 Iterator_facade: wrong return type</a></li>
<li><a class="reference" href="#iterator-facade-unclear-returns-clause-for-operator" id="id27" name="id27">9.26 Iterator_facade: unclear returns clause for operator[]</a></li>
<li><a class="reference" href="#iterator-facade-redundant-clause" id="id28" name="id28">9.27 Iterator_facade: redundant clause</a></li>
<li><a class="reference" href="#indirect-iterator-incorrect-specification-of-default-constructor" id="id29" name="id29">9.28 indirect_iterator: incorrect specification of default constructor</a></li>
<li><a class="reference" href="#indirect-iterator-unclear-specification-of-template-constructor" id="id30" name="id30">9.29 indirect_iterator: unclear specification of template constructor</a></li>
<li><a class="reference" href="#transform-iterator-argument-irregularity" id="id31" name="id31">9.30 transform_iterator argument irregularity</a></li>
<li><a class="reference" href="#function-output-iterator-overconstrained" id="id32" name="id32">9.31 function_output_iterator overconstrained</a></li>
<li><a class="reference" href="#should-output-proxy-really-be-a-named-type" id="id33" name="id33">9.32 Should output_proxy really be a named type?</a></li>
<li><a class="reference" href="#istreambuf-iterator-isn-t-a-readable-iterator" id="id34" name="id34">9.33 istreambuf_iterator isn't a Readable Iterator</a></li>
<li><a class="reference" href="#iterator-facade-free-functions-unspecified" id="id35" name="id35">9.34 iterator_facade free functions unspecified</a></li>
<li><a class="reference" href="#iterator-facade-too-many-equals" id="id36" name="id36">9.35 iterator_facade: too many equals?</a></li>
<li><a class="reference" href="#iterator-facade-function-requirements" id="id37" name="id37">9.36 iterator_facade function requirements</a></li>
</ul>
</li>
<li><a class="reference" href="#more-issues-not-from-matt-s-list" id="id38" name="id38">More Issues (not from Matt's list)</a><ul>
<li><a class="reference" href="#x-inheritance-in-iterator-adaptor-and-other-adaptors-is-an-overspecification" id="id39" name="id39">9.37x Inheritance in iterator_adaptor and other adaptors is an overspecification</a></li>
<li><a class="reference" href="#x-problem-with-specification-of-a-m-in-readable-iterator" id="id40" name="id40">9.38x Problem with specification of a->m in Readable Iterator</a></li>
<li><a class="reference" href="#x-counting-iterator-traversal-argument-unspecified" id="id41" name="id41">9.39x counting_iterator Traversal argument unspecified</a></li>
<li><a class="reference" href="#x-indirect-iterator-requirements-muddled" id="id42" name="id42">9.40x indirect_iterator requirements muddled</a></li>
<li><a class="reference" href="#x-problem-with-transform-iterator-requirements" id="id43" name="id43">9.41x Problem with transform_iterator requirements</a></li>
<li><a class="reference" href="#x-filter-iterator-details-unspecified" id="id44" name="id44">9.42x filter_iterator details unspecified</a></li>
<li><a class="reference" href="#x-transform-iterator-interoperability-too-restrictive" id="id45" name="id45">9.43x transform_iterator interoperability too restrictive</a></li>
<li><a class="reference" href="#y-indirect-iterator-and-smart-pointers" id="id46" name="id46">9.44y <tt class="literal"><span class="pre">indirect_iterator</span></tt> and smart pointers</a></li>
<li><a class="reference" href="#y-n1530-typos-and-editorial-changes-in-proposal-text-not-standardese" id="id47" name="id47">9.45y N1530: Typos and editorial changes in proposal text (not standardese)</a></li>
<li><a class="reference" href="#y-n1530-base-return-by-value-is-costly" id="id48" name="id48">9.46y N1530: <tt class="literal"><span class="pre">base()</span></tt> return-by-value is costly</a></li>
<li><a class="reference" href="#x-forgot-default-constructible-in-forward-traversal-iterator" id="id49" name="id49">9.47x Forgot default constructible in Forward Traversal Iterator</a></li>
<li><a class="reference" href="#x-editorial-changes-non-normative-text" id="id50" name="id50">9.48x Editorial changes (non-normative text)</a></li>
<li><a class="reference" href="#x-clarification-of-iterator-facade-requirements-and-type-members" id="id51" name="id51">9.49x Clarification of iterator_facade requirements and type members</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="issues-from-matt-s-tr-issues-list">
<h1><a class="toc-backref" href="#id1" name="issues-from-matt-s-tr-issues-list">Issues from Matt's TR issues list</a></h1>
<div class="section" id="iterator-access-overspecified">
<h2><a class="toc-backref" href="#id2" name="iterator-access-overspecified">9.1 iterator_access overspecified?</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The proposal includes:</p>
<pre class="literal-block">
enum iterator_access {
readable_iterator = 1, writable_iterator = 2,
swappable_iterator = 4, lvalue_iterator = 8
};
</pre>
<p>In general, the standard specifies thing like this as a bitmask
type with a list of defined names, and specifies neither the exact
type nor the specific values. Is there a reason for iterator_access
to be more specific?</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">The <tt class="literal"><span class="pre">iterator_access</span></tt> enum will be removed,
so this is no longer an issue. See the resolution to 9.15.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="operators-of-iterator-facade-overspecified">
<h2><a class="toc-backref" href="#id3" name="operators-of-iterator-facade-overspecified">9.2 operators of iterator_facade overspecified</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>In general, we've provided operational semantics for things like
operator++. That is, we've said that ++iter must work, without
requiring either a member function or a non-member function.
iterator_facade specifies most operators as member
functions. There's no inherent reason for these to be members, so
we should remove this requirement. Similarly, some operations are
specified as non-member functions but could be implemented as
members. Again, the standard doesn't make either of these choices,
and TR1 shouldn't, either. So: <tt class="literal"><span class="pre">operator*()</span></tt>, <tt class="literal"><span class="pre">operator++()</span></tt>,
<tt class="literal"><span class="pre">operator++(int)</span></tt>, <tt class="literal"><span class="pre">operator--()</span></tt>, <tt class="literal"><span class="pre">operator--(int)</span></tt>,
<tt class="literal"><span class="pre">operator+=</span></tt>, <tt class="literal"><span class="pre">operator-=</span></tt>, <tt class="literal"><span class="pre">operator-(difference_type)</span></tt>,
<tt class="literal"><span class="pre">operator-(iterator_facade</span> <span class="pre">instance)</span></tt>, and <tt class="literal"><span class="pre">operator+</span></tt> should
be specified with operational semantics and not explicitly required
to be members or non-members.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">Not a defect.</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body">The standard uses valid expressions such as <tt class="literal"><span class="pre">++iter</span></tt>
in requirements tables, such as for input iterator. However, for
classes, such as <tt class="literal"><span class="pre">reverse_iterator</span></tt>, the standard uses function
prototypes, as we have done here for
<tt class="literal"><span class="pre">iterator_facade</span></tt>. Further, the prototype specification does
not prevent the implementor from using members or non-members,
since nothing the user can do in a conforming program can detect
how the function is implemented.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="enable-if-interoperable-needs-standardese">
<h2><a class="toc-backref" href="#id4" name="enable-if-interoperable-needs-standardese">9.3 enable_if_interoperable needs standardese</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The only discussion of what this means is in a note, so is
non-normative. Further, the note seems to be incorrect. It says
that enable_if_interoperable only works for types that "are
interoperable, by which we mean they are convertible to each
other." This requirement is too strong: it should be that one of
the types is convertible to the other. N1541 48</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Add normative text. Relax requirements in the
proposed way.</p>
<p>Change:</p>
<blockquote>
[<em>Note:</em> The <tt class="literal"><span class="pre">enable_if_interoperable</span></tt> template used above is
for exposition purposes. The member operators should be only be
in an overload set provided the derived types <tt class="literal"><span class="pre">Dr1</span></tt> and
<tt class="literal"><span class="pre">Dr2</span></tt> are interoperable, by which we mean they are
convertible to each other. The <tt class="literal"><span class="pre">enable_if_interoperable</span></tt>
approach uses SFINAE to take the operators out of the overload
set when the types are not interoperable.]</blockquote>
<p>To:</p>
<blockquote class="last">
<p>The <tt class="literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for
exposition purposes. The member operators should only be in an
overload set provided the derived types <tt class="literal"><span class="pre">Dr1</span></tt> and <tt class="literal"><span class="pre">Dr2</span></tt> are
interoperable, meaning that at least one of the types is
convertible to the other. The <tt class="literal"><span class="pre">enable_if_interoperable</span></tt>
approach uses SFINAE to take the operators out of the overload
set when the types are not interoperable. The operators should
behave <em>as-if</em> <tt class="literal"><span class="pre">enable_if_interoperable</span></tt> were defined to be:</p>
<pre class="literal-block">
template <bool, typename> enable_if_interoperable_impl
{};
template <typename T> enable_if_interoperable_impl<true,T>
{ typedef T type; };
template<typename Dr1, typename Dr2, typename T>
struct enable_if_interoperable
: enable_if_interoperable_impl<
is_convertible<Dr1,Dr2>::value || is_convertible<Dr2,Dr1>::value
, T
>
{};
</pre>
</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="enable-if-convertible-unspecified-conflicts-with-requires">
<h2><a class="toc-backref" href="#id5" name="enable-if-convertible-unspecified-conflicts-with-requires">9.4 enable_if_convertible unspecified, conflicts with requires</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>In every place where enable_if_convertible is used it's used like
this (simplified):</p>
<pre class="literal-block">
template<class T>
struct C
{
template<class T1>
C(T1, enable_if_convertible<T1, T>::type* = 0);
};
</pre>
<p>The idea being that this constructor won't compile if T1 isn't
convertible to T. As a result, the constructor won't be considered
as a possible overload when constructing from an object x where the
type of x isn't convertible to T. In addition, however, each of
these constructors has a requires clause that requires
convertibility, so the behavior of a program that attempts such a
construction is undefined. Seems like the enable_if_convertible
part is irrelevant, and should be removed. There are two
problems. First, enable_if_convertible is never specified, so we
don't know what this is supposed to do. Second: we could reasonably
say that this overload should be disabled in certain cases or we
could reasonably say that behavior is undefined, but we can't say
both.</p>
<p>Thomas Witt writes that the goal of putting in
enable_if_convertible here is to make sure that a specific overload
doesn't interfere with the generic case except when that overload
makes sense. He agrees that what we currently have is deficient.
Dave Abrahams writes that there is no conflict with the requires
cause because the requires clause only takes effect when the
function is actually called. The presence of the constructor
signature can/will be detected by is_convertible without violating
the requires clause, and thus it makes a difference to disable
those constructor instantiations that would be disabled by
enable_if_convertible even if calling them invokes undefined
behavior. There was more discussion on the reflector:
c++std-lib-12312, c++std-lib-12325, c++std-lib- 12330,
c++std-lib-12334, c++std-lib-12335, c++std-lib-12336,
c++std-lib-12338, c++std-lib- 12362.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change:</p>
<blockquote>
[<em>Note:</em> The <tt class="literal"><span class="pre">enable_if_convertible<X,Y>::type</span></tt> expression
used in this section is for exposition purposes. The converting
constructors for specialized adaptors should be only be in an
overload set provided that an object of type <tt class="literal"><span class="pre">X</span></tt> is
implicitly convertible to an object of type <tt class="literal"><span class="pre">Y</span></tt>. The
<tt class="literal"><span class="pre">enable_if_convertible</span></tt> approach uses SFINAE to take the
constructor out of the overload set when the types are not
implicitly convertible.]</blockquote>
<p>To:</p>
<blockquote class="last">
<p>The <tt class="literal"><span class="pre">enable_if_convertible<X,Y>::type</span></tt> expression used in
this section is for exposition purposes. The converting
constructors for specialized adaptors should be only be in an
overload set provided that an object of type <tt class="literal"><span class="pre">X</span></tt> is
implicitly convertible to an object of type <tt class="literal"><span class="pre">Y</span></tt>. The
signatures involving <tt class="literal"><span class="pre">enable_if_convertible</span></tt> should behave
<em>as-if</em> <tt class="literal"><span class="pre">enable_if_convertible</span></tt> were defined to be:</p>
<pre class="literal-block">
template <bool> enable_if_convertible_impl
{};
template <> enable_if_convertible_impl<true>
{ struct type; };
template<typename From, typename To>
struct enable_if_convertible
: enable_if_convertible_impl<is_convertible<From,To>::value>
{};
</pre>
<p>If an expression other than the default argument is used to
supply the value of a function parameter whose type is written
in terms of <tt class="literal"><span class="pre">enable_if_convertible</span></tt>, the program is
ill-formed, no diagnostic required.</p>
<p>[<em>Note:</em> The <tt class="literal"><span class="pre">enable_if_convertible</span></tt> approach uses SFINAE to
take the constructor out of the overload set when the types are
not implicitly convertible. ]</p>
</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-adaptor-has-an-extraneous-bool-at-the-start-of-the-template-definition">
<h2><a class="toc-backref" href="#id6" name="iterator-adaptor-has-an-extraneous-bool-at-the-start-of-the-template-definition">9.5 iterator_adaptor has an extraneous 'bool' at the start of the template definition</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The title says it all; this is probably just a typo.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">Remove the 'bool'.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="name-of-private-member-shouldn-t-be-normative">
<h2><a class="toc-backref" href="#id7" name="name-of-private-member-shouldn-t-be-normative">9.6 Name of private member shouldn't be normative</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>iterator_adaptor has a private member named m_iterator. Presumably
this is for exposition only, since it's an implementation
detail. It needs to be marked as such.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><dl class="first">
<dt>Mark the member <tt class="literal"><span class="pre">m_iterator</span></tt> as exposition</dt>
<dd>only. Note/DWA: I think this is NAD because the user can't
detect it, though I'm happy to mark it exposition only.</dd>
</dl>
<p>In [lib.iterator.adaptor]</p>
<p>Change:</p>
<pre class="literal-block">
Base m_iterator;
</pre>
<p>to:</p>
<pre class="last literal-block">
Base m_iterator; // exposition only
</pre>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-adaptor-operations-specifications-are-a-bit-inconsistent">
<h2><a class="toc-backref" href="#id8" name="iterator-adaptor-operations-specifications-are-a-bit-inconsistent">9.7 iterator_adaptor operations specifications are a bit inconsistent</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>iterator_adpator() has a Requires clause, that Base must be default
constructible. iterator_adaptor(Base) has no Requires clause,
although the Returns clause says that the Base member is copy
construced from the argument (this may actually be an oversight in
N1550, which doesn't require iterators to be copy constructible or
assignable).</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Add a requirements section for the template
parameters of iterator_adaptor, and state that Base must be Copy
Constructible and Assignable.</p>
<p class="last">N1550 does in fact include requirements for copy constructible
and assignable in the requirements tables. To clarify, we've also
added the requirements to the text.</p>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="specialized-adaptors-text-should-be-normative">
<h2><a class="toc-backref" href="#id9" name="specialized-adaptors-text-should-be-normative">9.8 Specialized adaptors text should be normative</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>similar to 9.3, "Specialized Adaptors" has a note describing
enable_if_convertible. This should be normative text.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">Changed it to normative
text. See the resolution of 9.4</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="reverse-iterator-text-is-too-informal">
<h2><a class="toc-backref" href="#id10" name="reverse-iterator-text-is-too-informal">9.9 Reverse_iterator text is too informal</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>reverse iterator "flips the direction of the base iterator's
motion". This needs to be more formal, as in the current
standard. Something like: "iterates through the controlled sequence
in the opposite direction"</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change:</p>
<blockquote>
The reverse iterator adaptor flips the direction of a base
iterator's motion. Invoking <tt class="literal"><span class="pre">operator++()</span></tt> moves the base
iterator backward and invoking <tt class="literal"><span class="pre">operator--()</span></tt> moves the base
iterator forward.</blockquote>
<p>to:</p>
<blockquote class="last">
The reverse iterator adaptor iterates through the adapted iterator
range in the opposite direction.</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="prior-is-undefined">
<h2><a class="toc-backref" href="#id11" name="prior-is-undefined">9.10 'prior' is undefined</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>reverse_iterator::dereference is specified as calling a function
named 'prior' which has no specification.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change the specification to avoid using <tt class="literal"><span class="pre">prior</span></tt> as follows.</p>
<p>Remove:</p>
<pre class="literal-block">
typename reverse_iterator::reference dereference() const { return *prior(this->base()); }
</pre>
<p>And at the end of the operations section add:</p>
<blockquote class="last">
<p><tt class="literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"></td>
</tr>
</tbody>
</table>
<pre class="literal-block">
Iterator tmp = m_iterator;
return *--tmp;
</pre>
</blockquote>
</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body">The style of specification has changed because of issue 9.37x.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="in-other-words-is-bad-wording">
<h2><a class="toc-backref" href="#id12" name="in-other-words-is-bad-wording">9.11 "In other words" is bad wording</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>Transform iterator has a two-part specification: it does this, in
other words, it does that. "In other words" always means "I didn't
say it right, so I'll try again." We need to say it once.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change:</p>
<blockquote>
The transform iterator adapts an iterator by applying some function
object to the result of dereferencing the iterator. In other words,
the <tt class="literal"><span class="pre">operator*</span></tt> of the transform iterator first dereferences the
base iterator, passes the result of this to the function object, and
then returns the result.</blockquote>
<p>to:</p>
<blockquote class="last">
The transform iterator adapts an iterator by modifying the
<tt class="literal"><span class="pre">operator*</span></tt> to apply a function object to the result of
dereferencing the iterator and returning the result.</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="transform-iterator-shouldn-t-mandate-private-member">
<h2><a class="toc-backref" href="#id13" name="transform-iterator-shouldn-t-mandate-private-member">9.12 Transform_iterator shouldn't mandate private member</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>transform_iterator has a private member named 'm_f' which should be
marked "exposition only."</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Mark the member <tt class="literal"><span class="pre">m_f</span></tt> as exposition
only. Note/DWA: I think this is NAD because the user can't
detect it, though I'm happy to mark it exposition only.</p>
<p>Change:</p>
<pre class="literal-block">
UnaryFunction m_f;
</pre>
<p>to:</p>
<pre class="last literal-block">
UnaryFunction m_f; // exposition only
</pre>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="unclear-description-of-counting-iterator">
<h2><a class="toc-backref" href="#id14" name="unclear-description-of-counting-iterator">9.13 Unclear description of counting iterator</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The description of Counting iterator is unclear. "The counting
iterator adaptor implements dereference by returning a reference to
the base object. The other operations are implemented by the base
m_iterator, as per the inheritance from iterator_adaptor."</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change:</p>
<blockquote>
The counting iterator adaptor implements dereference by
returning a reference to the base object. The other operations
are implemented by the base <tt class="literal"><span class="pre">m_iterator</span></tt>, as per the
inheritance from <tt class="literal"><span class="pre">iterator_adaptor</span></tt>.</blockquote>
<p>to:</p>
<blockquote class="last">
<tt class="literal"><span class="pre">counting_iterator</span></tt> adapts an object by adding an
<tt class="literal"><span class="pre">operator*</span></tt> that returns the current value of the object. All
other iterator operations are forwarded to the adapted object.</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="counting-iterator-s-difference-type">
<h2><a class="toc-backref" href="#id15" name="counting-iterator-s-difference-type">9.14 Counting_iterator's difference type</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>Counting iterator has the following note:</p>
<blockquote>
[Note: implementers are encouraged to provide an implementation
of distance_to and a difference_type that avoids overflows in the
cases when the Incrementable type is a numeric type.]</blockquote>
<p>I'm not sure what this means. The user provides a template argument
named Difference, but there's no difference_type. I assume this is
just a glitch in the wording. But if implementors are encouraged to
ignore this argument if it won't work right, why is it there?</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">The <tt class="literal"><span class="pre">difference_type</span></tt> was inherited from
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>. However, we've removed the explicit
inheritance, so explicit typedefs have been added. See the
resolution of 9.37x.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="how-to-detect-lvalueness">
<h2><a class="toc-backref" href="#id16" name="how-to-detect-lvalueness">9.15 How to detect lvalueness?</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>Shortly after N1550 was accepted, we discovered that an iterator's
lvalueness can be determined knowing only its value_type. This
predicate can be calculated even for old-style iterators (on whose
reference type the standard places few requirements). A trait in
the Boost iterator library does it by relying on the compiler's
unwillingness to bind an rvalue to a T& function template
parameter. Similarly, it is possible to detect an iterator's
readability knowing only its value_type. Thus, any interface which
asks the user to explicitly describe an iterator's lvalue-ness or
readability seems to introduce needless complexity.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><ol class="first arabic simple">
<li>Remove the <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt> traits, and
remove the requirements in the Writable Iterator and Swappable
Iterator concepts that require their models to support these
traits.</li>
<li>Change the <tt class="literal"><span class="pre">is_readable</span></tt> specification. Remove the
requirement for support of the <tt class="literal"><span class="pre">is_readable</span></tt> trait from the
Readable Iterator concept.</li>
<li>Remove the <tt class="literal"><span class="pre">iterator_tag</span></tt> class and transplant the logic for
choosing an iterator category into <tt class="literal"><span class="pre">iterator_facade</span></tt>.</li>
<li>Change the specification of <tt class="literal"><span class="pre">traversal_category</span></tt>.</li>
<li>Remove Access parameters from N1530</li>
</ol>
<p>In N1550:</p>
<p>Remove:</p>
<blockquote>
<p>Since the access concepts are not related via refinement, but
instead cover orthogonal issues, we do not use tags for the
access concepts, but instead use the equivalent of a bit field.</p>
<p>We provide an access mechanism for mapping iterator types to
the new traversal tags and access bit field. Our design reuses
<tt class="literal"><span class="pre">iterator_traits<Iter>::iterator_category</span></tt> as the access
mechanism. To that end, the access and traversal information is
bundled into a single type using the following <cite>iterator_tag</cite>
class.</p>
<pre class="literal-block">
enum iterator_access { readable_iterator = 1, writable_iterator = 2,
swappable_iterator = 4, lvalue_iterator = 8 };
template <unsigned int access_bits, class TraversalTag>
struct iterator_tag : /* appropriate old category or categories */ {
static const iterator_access access =
(iterator_access)access_bits &
(readable_iterator | writable_iterator | swappable_iterator);
typedef TraversalTag traversal;
};
</pre>
<p>The <tt class="literal"><span class="pre">access_bits</span></tt> argument is declared to be <tt class="literal"><span class="pre">unsigned</span> <span class="pre">int</span></tt>
instead of the enum <tt class="literal"><span class="pre">iterator_access</span></tt> for convenience of
use. For example, the expression <tt class="literal"><span class="pre">(readable_iterator</span> <span class="pre">|</span>
<span class="pre">writable_iterator)</span></tt> produces an unsigned int, not an
<tt class="literal"><span class="pre">iterator_access</span></tt>. The purpose of the <tt class="literal"><span class="pre">lvalue_iterator</span></tt>
part of the <tt class="literal"><span class="pre">iterator_access</span></tt> enum is to communicate to
<tt class="literal"><span class="pre">iterator_tag</span></tt> whether the reference type is an lvalue so
that the appropriate old category can be chosen for the base
class. The <tt class="literal"><span class="pre">lvalue_iterator</span></tt> bit is not recorded in the
<tt class="literal"><span class="pre">iterator_tag::access</span></tt> data member.</p>
<p>The <tt class="literal"><span class="pre">iterator_tag</span></tt> class template is derived from the
appropriate iterator tag or tags from the old requirements
based on the access bits and traversal tag passed as template
parameters. The algorithm for determining the old tag or tags
picks the least refined old concepts that include all of the
requirements of the access and traversal concepts (that is, the
closest fit), if any such category exists. For example, the
category tag for a Readable Single Pass Iterator will always be
derived from <tt class="literal"><span class="pre">input_iterator_tag</span></tt>, while the category tag for
a Single Pass Iterator that is both Readable and Writable will
be derived from both <tt class="literal"><span class="pre">input_iterator_tag</span></tt> and
<tt class="literal"><span class="pre">output_iterator_tag</span></tt>.</p>
<p>We also provide several helper classes that make it convenient
to obtain the access and traversal characteristics of an
iterator. These helper classes work both for iterators whose
<tt class="literal"><span class="pre">iterator_category</span></tt> is <tt class="literal"><span class="pre">iterator_tag</span></tt> and also for
iterators using the original iterator categories.</p>
<pre class="literal-block">
template <class Iterator> struct is_readable { typedef ... type; };
template <class Iterator> struct is_writable { typedef ... type; };
template <class Iterator> struct is_swappable { typedef ... type; };
template <class Iterator> struct traversal_category { typedef ... type; };
</pre>
</blockquote>
<p>After:</p>
<blockquote>
Like the old iterator requirements, we provide tags for
purposes of dispatching based on the traversal concepts. The
tags are related via inheritance so that a tag is convertible
to another tag if the concept associated with the first tag is
a refinement of the second tag.</blockquote>
<p>Add:</p>
<blockquote>
<p>Our design reuses <tt class="literal"><span class="pre">iterator_traits<Iter>::iterator_category</span></tt>
to indicate an iterator's traversal capability. To specify
capabilities not captured by any old-style iterator category,
an iterator designer can use an <tt class="literal"><span class="pre">iterator_category</span></tt> type that
is convertible to both the the most-derived old iterator
category tag which fits, and the appropriate new iterator
traversal tag.</p>
<p>We do not provide tags for the purposes of dispatching based on
the access concepts, in part because we could not find a way to
automatically infer the right access tags for old-style
iterators. An iterator's writability may be dependent on the
assignability of its <tt class="literal"><span class="pre">value_type</span></tt> and there's no known way to
detect whether an arbitrary type is assignable. Fortunately,
the need for dispatching based on access capability is not as
great as the need for dispatching based on traversal
capability.</p>
</blockquote>
<p>From the Readable Iterator Requirements table, remove:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="37%" />
<col width="37%" />
<col width="26%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">is_readable<X>::type</span></tt></td>
<td><tt class="literal"><span class="pre">true_type</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>From the Writable Iterator Requirements table, remove:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="37%" />
<col width="21%" />
<col width="42%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">is_writable<X>::type</span></tt></td>
<td><tt class="literal"><span class="pre">true_type</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>From the Swappable Iterator Requirements table, remove:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="37%" />
<col width="19%" />
<col width="43%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">is_swappable<X>::type</span></tt></td>
<td><tt class="literal"><span class="pre">true_type</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>From [lib.iterator.synopsis] replace:</p>
<pre class="literal-block">
template <class Iterator> struct is_readable;
template <class Iterator> struct is_writable;
template <class Iterator> struct is_swappable;
template <class Iterator> struct traversal_category;
enum iterator_access { readable_iterator = 1, writable_iterator = 2,
swappable_iterator = 4, lvalue_iterator = 8 };
template <unsigned int access_bits, class TraversalTag>
struct iterator_tag : /* appropriate old category or categories */ {
static const iterator_access access =
(iterator_access)access_bits &
(readable_iterator | writable_iterator | swappable_iterator);
typedef TraversalTag traversal;
};
</pre>
<p>with:</p>
<pre class="literal-block">
template <class Iterator> struct is_readable_iterator;
template <class Iterator> struct iterator_traversal;
</pre>
<p>In [lib.iterator.traits], remove:</p>
<blockquote>
<p>The <tt class="literal"><span class="pre">iterator_tag</span></tt> class template is an iterator category tag
that encodes the access enum and traversal tag in addition to
being compatible with the original iterator tags. The
<tt class="literal"><span class="pre">iterator_tag</span></tt> class inherits from one of the original
iterator tags according to the following pseudo-code.</p>
<pre class="literal-block">
inherit-category(access, traversal-tag) =
if ((access & readable_iterator) && (access & lvalue_iterator)) {
if (traversal-tag is convertible to random_access_traversal_tag)
return random_access_iterator_tag;
else if (traversal-tag is convertible to bidirectional_traversal_tag)
return bidirectional_iterator_tag;
else if (traversal-tag is convertible to forward_traversal_tag)
return forward_iterator_tag;
else if (traversal-tag is convertible to single_pass_traversal_tag)
if (access-tag is convertible to writable_iterator_tag)
return input_output_iterator_tag;
else
return input_iterator_tag;
else
return null_category_tag;
} else if ((access & readable_iterator) and (access & writable_iterator)
and traversal-tag is convertible to single_pass_iterator_tag)
return input_output_iterator_tag;
else if (access & readable_iterator
and traversal-tag is convertible to single_pass_iterator_tag)
return input_iterator_tag;
else if (access & writable_iterator
and traversal-tag is convertible to incrementable_iterator_tag)
return output_iterator_tag;
else
return null_category_tag;
</pre>
<p>If the argument for <tt class="literal"><span class="pre">TraversalTag</span></tt> is not convertible to
<tt class="literal"><span class="pre">incrementable_iterator_tag</span></tt> then the program is ill-formed.</p>
</blockquote>
<p>Change:</p>
<blockquote>
<p>The <tt class="literal"><span class="pre">is_readable</span></tt>, <tt class="literal"><span class="pre">is_writable</span></tt>, <tt class="literal"><span class="pre">is_swappable</span></tt>, and
<tt class="literal"><span class="pre">traversal_category</span></tt> class templates are traits classes. For
iterators whose <tt class="literal"><span class="pre">iterator_traits<Iter>::iterator_category</span></tt>
type is <tt class="literal"><span class="pre">iterator_tag</span></tt>, these traits obtain the <tt class="literal"><span class="pre">access</span></tt>
enum and <tt class="literal"><span class="pre">traversal</span></tt> member type from within
<tt class="literal"><span class="pre">iterator_tag</span></tt>. For iterators whose
<tt class="literal"><span class="pre">iterator_traits<Iter>::iterator_category</span></tt> type is not
<tt class="literal"><span class="pre">iterator_tag</span></tt> and instead is a tag convertible to one of the
original tags, the appropriate traversal tag and access bits
are deduced. The following pseudo-code describes the
algorithm.</p>
<pre class="literal-block">
is-readable(Iterator) =
cat = iterator_traits<Iterator>::iterator_category;
if (cat == iterator_tag<Access,Traversal>)
return Access & readable_iterator;
else if (cat is convertible to input_iterator_tag)
return true;
else
return false;
is-writable(Iterator) =
cat = iterator_traits<Iterator>::iterator_category;
if (cat == iterator_tag<Access,Traversal>)
return Access & writable_iterator;
else if (cat is convertible to output_iterator_tag)
return true;
else if (
cat is convertible to forward_iterator_tag
and iterator_traits<Iterator>::reference is a
mutable reference)
return true;
else
return false;
is-swappable(Iterator) =
cat = iterator_traits<Iterator>::iterator_category;
if (cat == iterator_tag<Access,Traversal>)
return Access & swappable_iterator;
else if (cat is convertible to forward_iterator_tag) {
if (iterator_traits<Iterator>::reference is a const reference)
return false;
else
return true;
} else
return false;
traversal-category(Iterator) =
cat = iterator_traits<Iterator>::iterator_category;
if (cat == iterator_tag<Access,Traversal>)
return Traversal;
else if (cat is convertible to random_access_iterator_tag)
return random_access_traversal_tag;
else if (cat is convertible to bidirectional_iterator_tag)
return bidirectional_traversal_tag;
else if (cat is convertible to forward_iterator_tag)
return forward_traversal_tag;
else if (cat is convertible to input_iterator_tag)
return single_pass_iterator_tag;
else if (cat is convertible to output_iterator_tag)
return incrementable_iterator_tag;
else
return null_category_tag;
</pre>
<p>The following specializations provide the access and traversal
category tags for pointer types.</p>
<pre class="literal-block">
template <typename T>
struct is_readable<const T*> { typedef true_type type; };
template <typename T>
struct is_writable<const T*> { typedef false_type type; };
template <typename T>
struct is_swappable<const T*> { typedef false_type type; };
template <typename T>
struct is_readable<T*> { typedef true_type type; };
template <typename T>
struct is_writable<T*> { typedef true_type type; };
template <typename T>
struct is_swappable<T*> { typedef true_type type; };
template <typename T>
struct traversal_category<T*>
{
typedef random_access_traversal_tag type;
};
</pre>
</blockquote>
<p>to:</p>
<blockquote>
<p>The <tt class="literal"><span class="pre">is_readable_iterator</span></tt> class template satisfies the
UnaryTypeTrait requirements.</p>
<p>Given an iterator type <tt class="literal"><span class="pre">X</span></tt>,
<tt class="literal"><span class="pre">is_readable_iterator<X>::value</span></tt> yields <tt class="literal"><span class="pre">true</span></tt> if, for an
object <tt class="literal"><span class="pre">a</span></tt> of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">*a</span></tt> is convertible to
<tt class="literal"><span class="pre">iterator_traits<X>::value_type</span></tt>, and <tt class="literal"><span class="pre">false</span></tt> otherwise.</p>
<a class="target" id="category-to-traversal" name="category-to-traversal"></a><p><tt class="literal"><span class="pre">iterator_traversal<X>::type</span></tt> is</p>
<pre class="literal-block">
<em>category-to-traversal</em>(iterator_traits<X>::iterator_category)
</pre>
<p>where <em>category-to-traversal</em> is defined as follows</p>
<pre class="literal-block">
<em>category-to-traversal</em>(C) =
if (C is convertible to incrementable_traversal_tag)
return C;
else if (C is convertible to random_access_iterator_tag)
return random_access_traversal_tag;
else if (C is convertible to bidirectional_iterator_tag)
return bidirectional_traversal_tag;
else if (C is convertible to forward_iterator_tag)
return forward_traversal_tag;
else if (C is convertible to input_iterator_tag)
return single_pass_traversal_tag;
else if (C is convertible to output_iterator_tag)
return incrementable_traversal_tag;
else
<em>the program is ill-formed</em>
</pre>
</blockquote>
<p>In N1530:</p>
<p>In [lib.iterator.helper.synopsis]:</p>
<p>Change:</p>
<pre class="literal-block">
const unsigned use_default_access = -1;
struct iterator_core_access { /* implementation detail */ };
template <
class Derived
, class Value
, unsigned AccessCategory
, class TraversalCategory
, class Reference = Value&
, class Difference = ptrdiff_t
>
class iterator_facade;
template <
class Derived
, class Base
, class Value = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class iterator_adaptor;
template <
class Iterator
, class Value = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class indirect_iterator;
</pre>
<p>To:</p>
<pre class="literal-block">
struct iterator_core_access { /* implementation detail */ };
template <
class Derived
, class Value
, class CategoryOrTraversal
, class Reference = Value&
, class Difference = ptrdiff_t
>
class iterator_facade;
template <
class Derived
, class Base
, class Value = use_default
, class CategoryOrTraversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class iterator_adaptor;
template <
class Iterator
, class Value = use_default
, class CategoryOrTraversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class indirect_iterator;
</pre>
<p>Change:</p>
<pre class="literal-block">
template <
class Incrementable
, unsigned Access = use_default_access
, class Traversal = use_default
, class Difference = use_default
>
class counting_iterator
</pre>
<p>To:</p>
<pre class="literal-block">
template <
class Incrementable
, class CategoryOrTraversal = use_default
, class Difference = use_default
>
class counting_iterator;
</pre>
<p>In [lib.iterator.facade]:</p>
<p>Change:</p>
<pre class="literal-block">
template <
class Derived
, class Value
, unsigned AccessCategory
, class TraversalCategory
, class Reference = /* see below */
, class Difference = ptrdiff_t
>
class iterator_facade {
</pre>
<p>to:</p>
<pre class="literal-block">
template <
class Derived
, class Value
, class CategoryOrTraversal
, class Reference = Value&
, class Difference = ptrdiff_t
>
class iterator_facade {
</pre>
<p>Change:</p>
<pre class="literal-block">
typedef iterator_tag<AccessCategory, TraversalCategory> iterator_category;
</pre>
<p>to:</p>
<pre class="literal-block">
typedef /* see below */ iterator_category;
</pre>
<p>Change:</p>
<pre class="literal-block">
// Comparison operators
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type // exposition
operator ==(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator !=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator <(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator <=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator >(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator >=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator >=(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
// Iterator difference
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator -(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
// Iterator addition
template <class Derived, class V, class AC, class TC, class R, class D>
Derived operator+ (iterator_facade<Derived, V, AC, TC, R, D> const&,
typename Derived::difference_type n)
</pre>
<p>to:</p>
<pre class="literal-block">
// Comparison operators
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type // exposition
operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
// Iterator difference
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
/* see below */
operator-(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
// Iterator addition
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
typename Derived::difference_type n);
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (typename Derived::difference_type n,
iterator_facade<Dr,V,TC,R,D> const&);
</pre>
<p>After the <tt class="literal"><span class="pre">iterator_facade</span></tt> synopsis, add:</p>
<p>The <tt class="literal"><span class="pre">iterator_category</span></tt> member of <tt class="literal"><span class="pre">iterator_facade</span></tt> is</p>
<pre class="literal-block">
<em>iterator-category</em>(CategoryOrTraversal, value_type, reference)
</pre>
<p>where <em>iterator-category</em> is defined as follows:</p>
<pre class="last literal-block">
<em>iterator-category</em>(C,R,V) :=
if (C is convertible to std::input_iterator_tag
|| C is convertible to std::output_iterator_tag
)
return C
else if (C is not convertible to incrementable_traversal_tag)
<em>the program is ill-formed</em>
else return a type X satisfying the following two constraints:
1. X is convertible to X1, and not to any more-derived
type, where X1 is defined by:
if (R is a reference type
&& C is convertible to forward_traversal_tag)
{
if (C is convertible to random_access_traversal_tag)
X1 = random_access_iterator_tag
else if (C is convertible to bidirectional_traversal_tag)
X1 = bidirectional_iterator_tag
else
X1 = forward_iterator_tag
}
else
{
if (C is convertible to single_pass_traversal_tag
&& R is convertible to V)
X1 = input_iterator_tag
else
X1 = C
}
2. <a class="reference" href="#category-to-traversal"><em>category-to-traversal</em></a>(X) is convertible to the most
derived traversal tag type to which X is also
convertible, and not to any more-derived traversal tag
type.
</pre>
</td>
</tr>
</tbody>
</table>
<a class="target" id="iterator-category" name="iterator-category"></a><blockquote>
<p>In [lib.iterator.facade] <tt class="literal"><span class="pre">iterator_facade</span></tt> requirements:</p>
<p>Remove:</p>
<blockquote>
<tt class="literal"><span class="pre">AccessCategory</span></tt> must be an unsigned value which uses no more
bits than the greatest value of <tt class="literal"><span class="pre">iterator_access</span></tt>.</blockquote>
<p>In the <strong>Iterator Adaptor</strong> section, change:</p>
<blockquote>
Several of the template parameters of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> default
to <tt class="literal"><span class="pre">use_default</span></tt> (or <tt class="literal"><span class="pre">use_default_access</span></tt>).</blockquote>
<p>to:</p>
<blockquote>
Several of the template parameters of <tt class="literal"><span class="pre">iterator_adaptor</span></tt> default
to <tt class="literal"><span class="pre">use_default</span></tt>.</blockquote>
<p>In [lib.iterator.special.adaptors]:</p>
<p>Change:</p>
<pre class="literal-block">
template <
class Iterator
, class Value = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class indirect_iterator
</pre>
<p>to:</p>
<pre class="literal-block">
template <
class Iterator
, class Value = use_default
, class CategoryOrTraversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class indirect_iterator
</pre>
<p>Change:</p>
<pre class="literal-block">
template <
class Iterator2, class Value2, unsigned Access2, class Traversal2
, class Reference2, class Difference2
>
indirect_iterator(
</pre>
<p>to:</p>
<pre class="literal-block">
template <
class Iterator2, class Value2, class Category2
, class Reference2, class Difference2
>
indirect_iterator(
</pre>
<p>Change:</p>
<pre class="literal-block">
template <
class Incrementable
, unsigned Access = use_default_access
, class Traversal = use_default
, class Difference = use_default
>
class counting_iterator
</pre>
<p>to:</p>
<pre class="literal-block">
template <
class Incrementable
, class CategoryOrTraversal = use_default
, class Difference = use_default
>
class counting_iterator
</pre>
<p>Change:</p>
<pre class="literal-block">
typedef iterator_tag<
writable_iterator
, incrementable_traversal_tag
> iterator_category;
</pre>
<p>to:</p>
<blockquote>
typedef std::output_iterator_tag iterator_category;</blockquote>
<p>In [lib.iterator.adaptor]</p>
<p>Change:</p>
<pre class="literal-block">
template <
class Derived
, class Base
, class Value = use_default
, unsigned Access = use_default_access
, class Traversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class iterator_adaptor
</pre>
<p>To:</p>
<pre class="literal-block">
template <
class Derived
, class Base
, class Value = use_default
, class CategoryOrTraversal = use_default
, class Reference = use_default
, class Difference = use_default
>
class iterator_adaptor
</pre>
</blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body"></td>
</tr>
</tbody>
</table>
<ol class="arabic simple">
<li>There are two reasons for removing <tt class="literal"><span class="pre">is_writable</span></tt>
and <tt class="literal"><span class="pre">is_swappable</span></tt>. The first is that we do not know of
a way to fix the specification so that it gives the correct
answer for all iterators. Second, there was only a weak
motivation for having <tt class="literal"><span class="pre">is_writable</span></tt> and <tt class="literal"><span class="pre">is_swappable</span></tt>
there in the first place. The main motivation was simply
uniformity: we have tags for the old iterator categories
so we should have tags for the new iterator categories.
While having tags and the capability to dispatch based
on the traversal categories is often used, we see
less of a need for dispatching based on writability
and swappability, since typically algorithms
that need these capabilities have no alternative if
they are not provided.</li>
<li>We discovered that the <tt class="literal"><span class="pre">is_readable</span></tt> trait can be implemented
using only the iterator type itself and its <tt class="literal"><span class="pre">value_type</span></tt>.
Therefore we remove the requirement for <tt class="literal"><span class="pre">is_readable</span></tt> from the
Readable Iterator concept, and change the definition of
<tt class="literal"><span class="pre">is_readable</span></tt> so that it works for any iterator type.</li>
<li>The purpose of the <tt class="literal"><span class="pre">iterator_tag</span></tt> class was to bundle the
traversal and access category tags into the
<tt class="literal"><span class="pre">iterator_category</span></tt> typedef. With <tt class="literal"><span class="pre">is_writable</span></tt> and
<tt class="literal"><span class="pre">is_swappable</span></tt> gone, and <tt class="literal"><span class="pre">is_readable</span></tt> no longer in need of
special hints, there is no reason for iterators to provide
information about the access capabilities of an iterator. Thus
there is no need for the <tt class="literal"><span class="pre">iterator_tag</span></tt>. The traversal tag can
be directly used for the <tt class="literal"><span class="pre">iterator_category</span></tt>. If a new
iterator is intended to be backward compatible with old iterator
concepts, a tag type that is convertible to both one of the new
traversal tags and also to an old iterator tag can be created
and use for the <tt class="literal"><span class="pre">iterator_category</span></tt>.</li>
<li>The changes to the specification of <tt class="literal"><span class="pre">traversal_category</span></tt> are a
direct result of the removal of <tt class="literal"><span class="pre">iterator_tag</span></tt>.</li>
</ol>
</div>
<div class="section" id="is-writable-iterator-returns-false-positives">
<h2><a class="toc-backref" href="#id17" name="is-writable-iterator-returns-false-positives">9.16 is_writable_iterator returns false positives</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>is_writable_iterator returns false positives for forward iterators
whose value_type has a private assignment operator, or whose
reference type is not a reference (currently legal).</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed Resolution:</th></tr>
<tr><td> </td><td class="field-body">See the resolution to 9.15.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="is-swappable-iterator-returns-false-positives">
<h2><a class="toc-backref" href="#id18" name="is-swappable-iterator-returns-false-positives">9.17 is_swappable_iterator returns false positives</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>is_swappable_iterator has the same problems as
is_writable_iterator. In addition, if we allow users to write their
own iter_swap functions it's easy to imagine old-style iterators
for which is_swappable returns false negatives.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed Resolution:</th></tr>
<tr><td> </td><td class="field-body">See the resolution to 9.15.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="are-is-readable-is-writable-and-is-swappable-useful">
<h2><a class="toc-backref" href="#id19" name="are-is-readable-is-writable-and-is-swappable-useful">9.18 Are is_readable, is_writable, and is_swappable useful?</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>I am concerned that there is little use for any of is_readable,
is_writable, or is_swappable, and that not only do they unduly
constrain iterator implementors but they add overhead to
iterator_facade and iterator_adaptor in the form of a template
parameter which would otherwise be unneeded. Since we can't
implement two of them accurately for old-style iterators, I am
having a hard time justifying their impact on the rest of the
proposal(s).</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed Resolution:</th></tr>
<tr><td> </td><td class="field-body">See the resolution to 9.15.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="non-uniformity-of-the-lvalue-iterator-bit">
<h2><a class="toc-backref" href="#id20" name="non-uniformity-of-the-lvalue-iterator-bit">9.19 Non-Uniformity of the "lvalue_iterator Bit"</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The proposed iterator_tag class template accepts an "access bits"
parameter which includes a bit to indicate the iterator's
lvalueness (whether its dereference operator returns a reference to
its value_type. The relevant part of N1550 says:</p>
<blockquote>
The purpose of the lvalue_iterator part of the iterator_access
enum is to communicate to iterator_tagwhether the reference type
is an lvalue so that the appropriate old category can be chosen
for the base class. The lvalue_iterator bit is not recorded in
the iterator_tag::access data member.</blockquote>
<p>The lvalue_iterator bit is not recorded because N1550 aims to
improve orthogonality of the iterator concepts, and a new-style
iterator's lvalueness is detectable by examining its reference
type. This inside/outside difference is awkward and confusing.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed Resolution:</th></tr>
<tr><td> </td><td class="field-body">The iterator_tag class will be removed, so this is no longer an issue.
See the resolution to 9.15.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="traversal-concepts-and-tags">
<h2><a class="toc-backref" href="#id21" name="traversal-concepts-and-tags">9.20 Traversal Concepts and Tags</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>Howard Hinnant pointed out some inconsistencies with the naming of
these tag types:</p>
<pre class="literal-block">
incrementable_iterator_tag // ++r, r++
single_pass_iterator_tag // adds a == b, a != b
forward_traversal_iterator_tag // adds multi-pass
bidirectional_traversal_iterator_tag // adds --r, r--
random_access_traversal_iterator_tag // adds r+n,n+r,etc.
</pre>
<p>Howard thought that it might be better if all tag names contained
the word "traversal". It's not clear that would result in the best
possible names, though. For example, incrementable iterators can
only make a single pass over their input. What really distinguishes
single pass iterators from incrementable iterators is not that they
can make a single pass, but that they are equality
comparable. Forward traversal iterators really distinguish
themselves by introducing multi-pass capability. Without entering
a "Parkinson's Bicycle Shed" type of discussion, it might be worth
giving the names of these tags (and the associated concepts) some
extra attention.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change the names of the traversal tags to the
following names:</p>
<pre class="literal-block">
incrementable_traversal_tag
single_pass_traversal_tag
forward_traversal_tag
bidirectional_traversal_tag
random_access_traversal_tag
</pre>
<p>In [lib.iterator.traversal]:</p>
<p>Change:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="39%" />
<col width="37%" />
<col width="24%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">traversal_category<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">incrementable_iterator_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="39%" />
<col width="37%" />
<col width="24%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">iterator_traversal<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">incrementable_traversal_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>Change:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="36%" />
<col width="33%" />
<col width="31%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">traversal_category<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">single_pass_iterator_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="36%" />
<col width="33%" />
<col width="31%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">iterator_traversal<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">single_pass_traversal_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>Change:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="44%" />
<col width="39%" />
<col width="17%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">traversal_category<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">forward_traversal_iterator_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="38%" />
<col width="34%" />
<col width="27%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">iterator_traversal<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">forward_traversal_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>Change:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="35%" />
<col width="44%" />
<col width="21%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">traversal_category<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">bidirectional_traversal_iterator_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="38%" />
<col width="37%" />
<col width="25%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">iterator_traversal<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">bidirectional_traversal_tag</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>Change:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="31%" />
<col width="35%" />
<col width="18%" />
<col width="16%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">traversal_category<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">random_access_traversal_iterator_tag</span></tt></td>
<td> </td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="28%" />
<col width="30%" />
<col width="23%" />
<col width="20%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">iterator_traversal<X>::type</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">random_access_traversal_tag</span></tt></td>
<td> </td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>In [lib.iterator.synopsis], change:</p>
<pre class="literal-block">
struct incrementable_iterator_tag { };
struct single_pass_iterator_tag : incrementable_iterator_tag { };
struct forward_traversal_tag : single_pass_iterator_tag { };
</pre>
<p>to:</p>
<pre class="literal-block">
struct incrementable_traversal_tag { };
struct single_pass_traversal_tag : incrementable_traversal_tag { };
struct forward_traversal_tag : single_pass_traversal_tag { };
</pre>
<p>Remove:</p>
<pre class="last literal-block">
struct null_category_tag { };
struct input_output_iterator_tag : input_iterator_tag, output_iterator_tag {};
</pre>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-facade-derived-template-argument-underspecified">
<h2><a class="toc-backref" href="#id22" name="iterator-facade-derived-template-argument-underspecified">9.21 iterator_facade Derived template argument underspecified</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The first template argument to iterator_facade is named Derived,
and the proposal says:</p>
<blockquote>
The Derived template parameter must be a class derived from
iterator_facade.</blockquote>
<p>First, iterator_facade is a template, so cannot be derived
from. Rather, the class must be derived from a specialization of
iterator_facade. More important, isn't Derived required to be the
class that is being defined? That is, if I understand it right, the
definition of D here this is not valid:</p>
<pre class="literal-block">
class C : public iterator_facade<C, ... > { ... };
class D : public iterator_facade<C, ...> { ... };
</pre>
<p>In the definition of D, the Derived argument to iterator_facade is
a class derived from a specialization of iterator_facade, so the
requirement is met. Shouldn't the requirement be more like "when
using iterator_facade to define an iterator class Iter, the class
Iter must be derived from a specialization of iterator_facade whose
first template argument is Iter." That's a bit awkward, but at the
moment I don't see a better way of phrasing it.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">In [lib.iterator.facade]</p>
<p>Remove:</p>
<blockquote>
The <tt class="literal"><span class="pre">Derived</span></tt> template parameter must be a class derived from
<tt class="literal"><span class="pre">iterator_facade</span></tt>.</blockquote>
<p>Change:</p>
<blockquote>
The following table describes the other requirements on the
<tt class="literal"><span class="pre">Derived</span></tt> parameter. Depending on the resulting iterator's
<tt class="literal"><span class="pre">iterator_category</span></tt>, a subset of the expressions listed in the table
are required to be valid. The operations in the first column must be
accessible to member functions of class <tt class="literal"><span class="pre">iterator_core_access</span></tt>.</blockquote>
<p>to:</p>
<blockquote>
The following table describes the typical valid expressions on
<tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">Derived</span></tt> parameter, depending on the
iterator concept(s) it will model. The operations in the first
column must be made accessible to member functions of class
<tt class="literal"><span class="pre">iterator_core_access</span></tt>. In addition,
<tt class="literal"><span class="pre">static_cast<Derived*>(iterator_facade*)</span></tt> shall be well-formed.</blockquote>
<p>In [lib.iterator.adaptor]</p>
<p>Change:</p>
<blockquote>
The <tt class="literal"><span class="pre">iterator_adaptor</span></tt> is a base class template derived from
an instantiation of <tt class="literal"><span class="pre">iterator_facade</span></tt>.</blockquote>
<p>to:</p>
<blockquote>
Each specialization of the <tt class="literal"><span class="pre">iterator_adaptor</span></tt> class template
is derived from a specialization of <tt class="literal"><span class="pre">iterator_facade</span></tt>.</blockquote>
<p>Change:</p>
<blockquote>
The <tt class="literal"><span class="pre">Derived</span></tt> template parameter must be a derived class of
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>.</blockquote>
<p>To:</p>
<blockquote class="last">
<tt class="literal"><span class="pre">static_cast<Derived*>(iterator_adaptor*)</span></tt> shall be well-formed.</blockquote>
</td>
</tr>
</tbody>
</table>
<p>[Note: The proposed resolution to Issue 9.37 contains related
changes]</p>
</div>
<div class="section" id="return-type-of-iterator-difference-for-iterator-facade">
<h2><a class="toc-backref" href="#id23" name="return-type-of-iterator-difference-for-iterator-facade">9.22 return type of Iterator difference for iterator facade</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The proposal says:</p>
<pre class="literal-block">
template <class Dr1, class V1, class AC1, class TC1, class R1, class D1,
class Dr2, class V2, class AC2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1, Dr2, bool>::type
operator -(iterator_facade<Dr1, V1, AC1, TC1, R1, D1> const& lhs,
iterator_facade<Dr2, V2, AC2, TC2, R2, D2> const& rhs);
</pre>
<p>Shouldn't the return type be one of the two iterator types? Which
one? The idea is that if one of the iterator types can be converted
to the other type, then the subtraction is okay. Seems like the
return type should then be the type that was converted to. Is that
right?</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">See resolution to 9.34.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-facade-minor-wording-issue">
<h2><a class="toc-backref" href="#id24" name="iterator-facade-minor-wording-issue">9.23 Iterator_facade: minor wording Issue</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>In the table that lists the required (sort of) member functions of
iterator types that are based on iterator_facade, the entry for
c.equal(y) says:</p>
<blockquote>
true iff c and y refer to the same position. Implements c == y
and c != y. The second sentence is inside out. c.equal(y) does
not implement either of these operations. It is used to implement
them. Same thing in the description of c.distance_to(z).</blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">remove "implements" descriptions from
table. See resolution to 9.34</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="use-of-undefined-name-in-iterator-facade-table">
<h2><a class="toc-backref" href="#id25" name="use-of-undefined-name-in-iterator-facade-table">9.24 Use of undefined name in iterator_facade table</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>Several of the descriptions use the name X without defining
it. This seems to be a carryover from the table immediately above
this section, but the text preceding that table says "In the table
below, X is the derived iterator type." Looks like the X::
qualifiers aren't really needed; X::reference can simply be
reference, since that's defined by the iterator_facade
specialization itself.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Remove references to X.</p>
<p>In [lib.iterator.facade] operations <tt class="literal"><span class="pre">operator->()</span> <span class="pre">const;</span></tt>:</p>
<blockquote class="last">
<p>Change:</p>
<blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">If <tt class="literal"><span class="pre">X::reference</span></tt> is a reference type, an object
of type <tt class="literal"><span class="pre">X::pointer</span></tt> equal to:</p>
<pre class="literal-block">
&static_cast<Derived const*>(this)->dereference()
</pre>
<p>Otherwise returns an object of unspecified type such that,
given an object <tt class="literal"><span class="pre">a</span></tt> of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">a->m</span></tt> is equivalent
to <tt class="literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">*a,</span> <span class="pre">w.m)</span></tt> for some temporary object <tt class="literal"><span class="pre">w</span></tt> of type
<tt class="literal"><span class="pre">X::value_type</span></tt>.</p>
<p class="last">The type <tt class="literal"><span class="pre">X::pointer</span></tt> is <tt class="literal"><span class="pre">Value*</span></tt> if
<tt class="literal"><span class="pre">is_writable_iterator<X>::value</span></tt> is <tt class="literal"><span class="pre">true</span></tt>, and
<tt class="literal"><span class="pre">Value</span> <span class="pre">const*</span></tt> otherwise.</p>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">If <tt class="literal"><span class="pre">reference</span></tt> is a reference type, an object
of type <tt class="literal"><span class="pre">pointer</span></tt> equal to:</p>
<pre class="literal-block">
&static_cast<Derived const*>(this)->dereference()
</pre>
<p class="last">Otherwise returns an object of unspecified type such that,
<tt class="literal"><span class="pre">(*static_cast<Derived</span> <span class="pre">const*>(this))->m</span></tt> is equivalent
to <tt class="literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">**static_cast<Derived</span> <span class="pre">const*>(this),</span> <span class="pre">w.m)</span></tt> for
some temporary object <tt class="literal"><span class="pre">w</span></tt> of type <tt class="literal"><span class="pre">value_type</span></tt>.</p>
</td>
</tr>
</tbody>
</table>
</blockquote>
<p>Further changes are covered by issue 9.26.</p>
</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-facade-wrong-return-type">
<h2><a class="toc-backref" href="#id26" name="iterator-facade-wrong-return-type">9.25 Iterator_facade: wrong return type</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>Several of the member functions return a Derived object or a
Derived&. Their Effects clauses end with:</p>
<pre class="literal-block">
return *this;
</pre>
<p>This should be</p>
<pre class="literal-block">
return *static_cast<Derived*>(this);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">In [lib.iterator.facade], in the effects clause
of the following operations:</p>
<pre class="literal-block">
Derived& operator++()
Derived& operator--()
Derived& operator+=(difference_type n)
Derived& operator-=(difference_type n)
</pre>
<dl class="last">
<dt>Change:</dt>
<dd><tt class="literal"><span class="pre">return</span> <span class="pre">*this</span></tt></dd>
<dt>to:</dt>
<dd><tt class="literal"><span class="pre">return</span> <span class="pre">*static_cast<Derived*>(this);</span></tt></dd>
</dl>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-facade-unclear-returns-clause-for-operator">
<h2><a class="toc-backref" href="#id27" name="iterator-facade-unclear-returns-clause-for-operator">9.26 Iterator_facade: unclear returns clause for operator[]</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The returns clause for <tt class="literal"><span class="pre">operator[](difference_type</span> <span class="pre">n)</span></tt> const
says:</p>
<blockquote>
Returns: an object convertible to X::reference and holding a copy
p of a+n such that, for a constant object v of type
X::value_type, X::reference(a[n] = v) is equivalent to p = v.
This needs to define 'a', but assuming it's supposed to be
<tt class="literal"><span class="pre">*this</span></tt> (or maybe <tt class="literal"><span class="pre">*(Derived*)this</span></tt>), it still isn't clear
what this says. Presumably, the idea is that you can index off of
an iterator and assign to the result. But why the requirement
that it hold a copy of a+n? Granted, that's probably how it's
implemented, but it seems over-constrained. And the last phrase
seems wrong. p is an iterator; there's no requirement that you
can assign a value_type object to it. Should that be <tt class="literal"><span class="pre">*p</span> <span class="pre">=</span> <span class="pre">v</span></tt>?
But why the cast in reference(a[n] = v)?</blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">In section operator[]:</p>
<blockquote>
<p>Change:</p>
<blockquote>
Writable iterators built with <tt class="literal"><span class="pre">iterator_facade</span></tt> implement
the semantics required by the preferred resolution to <cite>issue
299</cite> and adopted by proposal <cite>n1477</cite>: the result of <tt class="literal"><span class="pre">p[n]</span></tt>
is a proxy object containing a copy of <tt class="literal"><span class="pre">p+n</span></tt>, and <tt class="literal"><span class="pre">p[n]</span> <span class="pre">=</span>
<span class="pre">x</span></tt> is equivalent to <tt class="literal"><span class="pre">*(p</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">x</span></tt>. This approach will
work properly for any random-access iterator regardless of
the other details of its implementation. A user who knows
more about the implementation of her iterator is free to
implement an <tt class="literal"><span class="pre">operator[]</span></tt> which returns an lvalue in the
derived iterator class; it will hide the one supplied by
<tt class="literal"><span class="pre">iterator_facade</span></tt> from clients of her iterator.</blockquote>
<p>to:</p>
<blockquote>
Writable iterators built with <tt class="literal"><span class="pre">iterator_facade</span></tt> implement
the semantics required by the preferred resolution to <cite>issue
299</cite> and adopted by proposal <cite>n1550</cite>: the result of <tt class="literal"><span class="pre">p[n]</span></tt>
is an object convertible to the iterator's <tt class="literal"><span class="pre">value_type</span></tt>,
and <tt class="literal"><span class="pre">p[n]</span> <span class="pre">=</span> <span class="pre">x</span></tt> is equivalent to <tt class="literal"><span class="pre">*(p</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">x</span></tt> (Note:
This result object may be implemented as a proxy containing a
copy of <tt class="literal"><span class="pre">p+n</span></tt>). This approach will work properly for any
random-access iterator regardless of the other details of its
implementation. A user who knows more about the
implementation of her iterator is free to implement an
<tt class="literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator
class; it will hide the one supplied by <tt class="literal"><span class="pre">iterator_facade</span></tt>
from clients of her iterator.</blockquote>
</blockquote>
<p>In [lib.iterator.facade] operations:</p>
<blockquote class="last">
<p>Change:</p>
<blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">an object convertible to <tt class="literal"><span class="pre">X::reference</span></tt> and
holding a copy <em>p</em> of <tt class="literal"><span class="pre">a+n</span></tt> such that, for a constant
object <tt class="literal"><span class="pre">v</span></tt> of type <tt class="literal"><span class="pre">X::value_type</span></tt>, <tt class="literal"><span class="pre">X::reference(a[n]</span>
<span class="pre">=</span> <span class="pre">v)</span></tt> is equivalent to <tt class="literal"><span class="pre">p</span> <span class="pre">=</span> <span class="pre">v</span></tt>.</td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">an object convertible to <tt class="literal"><span class="pre">value_type</span></tt>. For
constant objects <tt class="literal"><span class="pre">v</span></tt> of type <tt class="literal"><span class="pre">value_type</span></tt>, and <tt class="literal"><span class="pre">n</span></tt> of
type <tt class="literal"><span class="pre">difference_type</span></tt>, <tt class="literal"><span class="pre">(*this)[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt> is equivalent
to <tt class="literal"><span class="pre">*(*this</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt>, and <tt class="literal"><span class="pre">static_cast<value_type</span>
<span class="pre">const&>((*this)[n])</span></tt> is equivalent to
<tt class="literal"><span class="pre">static_cast<value_type</span> <span class="pre">const&>(*(*this</span> <span class="pre">+</span> <span class="pre">n))</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-facade-redundant-clause">
<h2><a class="toc-backref" href="#id28" name="iterator-facade-redundant-clause">9.27 Iterator_facade: redundant clause</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">operator-</span></tt> has both an effects clause and a returns
clause. Looks like the returns clause should be removed.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Remove the returns clause.</p>
<p>In [lib.iterator.facade] operations:</p>
<dl class="last">
<dt>Remove:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">static_cast<Derived</span> <span class="pre">const*>(this)->advance(-n);</span></tt></td>
</tr>
</tbody>
</table>
</dd>
</dl>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="indirect-iterator-incorrect-specification-of-default-constructor">
<h2><a class="toc-backref" href="#id29" name="indirect-iterator-incorrect-specification-of-default-constructor">9.28 indirect_iterator: incorrect specification of default constructor</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The default constructor returns "An instance of indirect_iterator
with a default constructed base object", but the constructor that
takes an Iterator object returns "An instance of indirect_iterator
with the iterator_adaptor subobject copy constructed from x." The
latter is the correct form, since it does not reach inside the base
class for its semantics. So the default constructor shoudl return
"An instance of indirect_iterator with a default-constructed
iterator_adaptor subobject."</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><dl class="first last">
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> with
a default constructed base object.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> with
a default-constructed <tt class="literal"><span class="pre">m_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
</dl>
</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body">Inheritance from iterator_adaptor has been removed, so we instead
give the semantics in terms of the (exposition only) member
<tt class="literal"><span class="pre">m_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="indirect-iterator-unclear-specification-of-template-constructor">
<h2><a class="toc-backref" href="#id30" name="indirect-iterator-unclear-specification-of-template-constructor">9.29 indirect_iterator: unclear specification of template constructor</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The templated constructor that takes an indirect_iterator with a
different set of template arguments says that it returns "An
instance of indirect_iterator that is a copy of [the argument]".
But the type of the argument is different from the type of the
object being constructed, and there is no description of what
a "copy" means. The Iterator template parameter for the argument
must be convertible to the Iterator template parameter for the type
being constructed, which suggests that the argument's contained
Iterator object should be converted to the target type's Iterator
type. Is that what's meant here?
(Pete later writes: In fact, this problem is present in all of the
specialized adaptors that have a constructor like this: the
constructor returns "a copy" of the argument without saying what a
copy is.)</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><dl class="first last">
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> that is a copy of <tt class="literal"><span class="pre">y</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> whose
<tt class="literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="literal"><span class="pre">y.base()</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
</dl>
</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body">Inheritance from iterator_adaptor has been removed, so we
instead give the semantics in terms of the member <tt class="literal"><span class="pre">m_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="transform-iterator-argument-irregularity">
<h2><a class="toc-backref" href="#id31" name="transform-iterator-argument-irregularity">9.30 transform_iterator argument irregularity</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>The specialized adaptors that take both a Value and a Reference
template argument all take them in that order, i.e. Value precedes
Reference in the template argument list, with the exception of
transform_iterator, where Reference precedes Value. This seems like
a possible source of confusion. Is there a reason why this order is
preferable?</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">NAD</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body">defaults for Value depend on Reference. A sensible
Value can almost always be computed from Reference. The first
parameter is UnaryFunction, so the argument order is already
different from the other adapters.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="function-output-iterator-overconstrained">
<h2><a class="toc-backref" href="#id32" name="function-output-iterator-overconstrained">9.31 function_output_iterator overconstrained</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>function_output_iterator requirements says: "The UnaryFunction must
be Assignable, Copy Constructible, and the expression f(x) must be
valid, where f is an object of type UnaryFunction and x is an
object of a type accepted by f."</p>
<p>Everything starting with "and," somewhat reworded, is actually a
constraint on output_proxy::operator=. All that's needed to create
a function_output_iterator object is that the UnaryFunction type be
Assignable and CopyConstructible. That's also sufficient to
dereference and to increment such an object. It's only when you try
to assign through a dereferenced iterator that f(x) has to work,
and then only for the particular function object that the iterator
holds and for the particular value that is being assigned.</p>
<dl>
<dt>Addition from Jeremy:</dt>
<dd>The constructor for <tt class="literal"><span class="pre">function_output_iterator</span></tt> is also
slightly overconstrained because it requires
the <tt class="literal"><span class="pre">UnaryFunction</span></tt> to have a default constructor
even when the default constructor of <tt class="literal"><span class="pre">function_output_iterator</span></tt>
is not used.</dd>
</dl>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><dl class="first">
<dt>Change:</dt>
<dd><tt class="literal"><span class="pre">output_proxy</span> <span class="pre">operator*();</span></tt></dd>
<dt>to:</dt>
<dd><tt class="literal"><span class="pre">/*</span> <span class="pre">see</span> <span class="pre">below</span> <span class="pre">*/</span> <span class="pre">operator*();</span></tt></dd>
</dl>
<p>After <tt class="literal"><span class="pre">function_output_iterator&</span> <span class="pre">operator++(int);</span></tt> add:</p>
<pre class="literal-block">
private:
UnaryFunction m_f; // exposition only
</pre>
<dl>
<dt>Change:</dt>
<dd>The <tt class="literal"><span class="pre">UnaryFunction</span></tt> must be Assignable, Copy Constructible,
and the expression <tt class="literal"><span class="pre">f(x)</span></tt> must be valid, where <tt class="literal"><span class="pre">f</span></tt> is an
object of type <tt class="literal"><span class="pre">UnaryFunction</span></tt> and <tt class="literal"><span class="pre">x</span></tt> is an object of a
type accepted by <tt class="literal"><span class="pre">f</span></tt>. The resulting
<tt class="literal"><span class="pre">function_output_iterator</span></tt> is a model of the Writable and
Incrementable Iterator concepts.</dd>
<dt>to:</dt>
<dd><tt class="literal"><span class="pre">UnaryFunction</span></tt> must be Assignable and Copy Constructible.</dd>
</dl>
<p class="last">After the requirements section, add:</p>
</td>
</tr>
</tbody>
</table>
<div class="topic">
<p class="topic-title"><tt class="literal"><span class="pre">function_output_iterator</span></tt> models</p>
<blockquote>
<tt class="literal"><span class="pre">function_output_iterator</span></tt> is a model of the Writable and
Incrementable Iterator concepts.</blockquote>
<dl>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">function_output_iterator</span></tt> with
<tt class="literal"><span class="pre">f</span></tt> stored as a data member.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="literal"><span class="pre">function_output_iterator</span></tt>
with <tt class="literal"><span class="pre">m_f</span></tt> constructed from <tt class="literal"><span class="pre">f</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><p class="first"><tt class="literal"><span class="pre">output_proxy</span> <span class="pre">operator*();</span></tt></p>
<table class="last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">output_proxy</span></tt> constructed with
a copy of the unary function <tt class="literal"><span class="pre">f</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><p class="first"><tt class="literal"><span class="pre">operator*();</span></tt></p>
<table class="last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An object <tt class="literal"><span class="pre">r</span></tt> of unspecified type such that <tt class="literal"><span class="pre">r</span> <span class="pre">=</span> <span class="pre">t</span></tt>
is equivalent to <tt class="literal"><span class="pre">m_f(t)</span></tt> for all <tt class="literal"><span class="pre">t</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Remove:</dt>
<dd><p class="first"><tt class="literal"><span class="pre">function_output_iterator::output_proxy</span></tt> operations</p>
<p><tt class="literal"><span class="pre">output_proxy(UnaryFunction&</span> <span class="pre">f);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">output_proxy</span></tt> with <tt class="literal"><span class="pre">f</span></tt> stored as
a data member.</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">template</span> <span class="pre"><class</span> <span class="pre">T></span> <span class="pre">output_proxy&</span> <span class="pre">operator=(const</span> <span class="pre">T&</span> <span class="pre">value);</span></tt></p>
<table class="last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
m_f(value);
return *this;
</pre>
</td>
</tr>
</tbody>
</table>
</dd>
</dl>
<p>Change:</p>
<pre class="literal-block">
explicit function_output_iterator(const UnaryFunction& f = UnaryFunction());
</pre>
<p>to:</p>
<pre class="literal-block">
explicit function_output_iterator();
explicit function_output_iterator(const UnaryFunction& f);
</pre>
</div>
</div>
<div class="section" id="should-output-proxy-really-be-a-named-type">
<h2><a class="toc-backref" href="#id33" name="should-output-proxy-really-be-a-named-type">9.32 Should output_proxy really be a named type?</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>This means someone can store an output_proxy object for later use,
whatever that means. It also constrains output_proxy to hold a copy
of the function object, rather than a pointer to the iterator
object. Is all this mechanism really necessary?</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">See issue 9.31.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="istreambuf-iterator-isn-t-a-readable-iterator">
<h2><a class="toc-backref" href="#id34" name="istreambuf-iterator-isn-t-a-readable-iterator">9.33 istreambuf_iterator isn't a Readable Iterator</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12333:</p>
<blockquote>
N1550 requires that for a Readable Iterator a of type X, <tt class="literal"><span class="pre">*a</span></tt>
returns an object of type
<tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt>. <tt class="literal"><span class="pre">istreambuf_iterator::operator*</span></tt>
returns <tt class="literal"><span class="pre">charT</span></tt>, but <tt class="literal"><span class="pre">istreambuf_iterator::reference</span></tt> is
<tt class="literal"><span class="pre">charT&</span></tt>. So am I overlooking something, or is
<tt class="literal"><span class="pre">istreambuf_iterator</span></tt> not Readable.</blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Remove all constraints on
<tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt> in Readable Iterator and Lvalue
Iterator. Change Lvalue Iterator to refer to <tt class="literal"><span class="pre">T&</span></tt> instead of
<tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt>.</p>
<dl>
<dt>Change:</dt>
<dd>A class or built-in type <tt class="literal"><span class="pre">X</span></tt> models the <em>Readable Iterator</em>
concept for the value type <tt class="literal"><span class="pre">T</span></tt> if the following expressions
are valid and respect the stated semantics. <tt class="literal"><span class="pre">U</span></tt> is the type
of any specified member of type <tt class="literal"><span class="pre">T</span></tt>.</dd>
<dt>to:</dt>
<dd>A class or built-in type <tt class="literal"><span class="pre">X</span></tt> models the <em>Readable Iterator</em>
concept for value type <tt class="literal"><span class="pre">T</span></tt> if, in addition to <tt class="literal"><span class="pre">X</span></tt> being
Assignable and Copy Constructible, the following expressions
are valid and respect the stated semantics. <tt class="literal"><span class="pre">U</span></tt> is the type
of any specified member of type <tt class="literal"><span class="pre">T</span></tt>.</dd>
</dl>
<p>From the Input Iterator Requirements table, remove:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="37%" />
<col width="37%" />
<col width="26%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt></td>
<td>Convertible to
<tt class="literal"><span class="pre">iterator_traits<X>::value_type</span></tt></td>
<td> </td>
</tr>
</tbody>
</table>
</blockquote>
<p>Change:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="37%" />
<col width="37%" />
<col width="26%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">*a</span></tt></td>
<td><tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt></td>
<td>pre: <tt class="literal"><span class="pre">a</span></tt> is
dereferenceable. If <tt class="literal"><span class="pre">a</span>
<span class="pre">==</span> <span class="pre">b</span></tt> then <tt class="literal"><span class="pre">*a</span></tt> is
equivalent to <tt class="literal"><span class="pre">*b</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="28%" />
<col width="20%" />
<col width="52%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">*a</span></tt></td>
<td>Convertible to <tt class="literal"><span class="pre">T</span></tt></td>
<td><dl class="first last">
<dt>pre: <tt class="literal"><span class="pre">a</span></tt> is dereferenceable. If <tt class="literal"><span class="pre">a</span> <span class="pre">==</span> <span class="pre">b</span></tt> then <tt class="literal"><span class="pre">*a</span></tt></dt>
<dd>is equivalent to <tt class="literal"><span class="pre">*b</span></tt>.</dd>
</dl>
</td>
</tr>
</tbody>
</table>
</blockquote>
<dl>
<dt>Change:</dt>
<dd>The <em>Lvalue Iterator</em> concept adds the requirement that the
<tt class="literal"><span class="pre">reference</span></tt> type be a reference to the value type of the
iterator.</dd>
<dt>to:</dt>
<dd>The <em>Lvalue Iterator</em> concept adds the requirement that the
return type of <tt class="literal"><span class="pre">operator*</span></tt> type be a reference to the value
type of the iterator.</dd>
</dl>
<p>Change:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="42%" />
<col width="14%" />
<col width="44%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="3">Lvalue Iterator Requirements</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">iterator_traits<X>::reference</span></tt></td>
<td><tt class="literal"><span class="pre">T&</span></tt></td>
<td><tt class="literal"><span class="pre">T</span></tt> is <em>cv</em>
<tt class="literal"><span class="pre">iterator_traits<X>::value_type</span></tt>
where <em>cv</em> is an optional
cv-qualification</td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="22%" />
<col width="19%" />
<col width="59%" />
</colgroup>
<thead valign="bottom">
<tr><th colspan="3">Lvalue Iterator Requirements</th>
</tr>
<tr><th>Expression</th>
<th>Return Type</th>
<th>Note/Assertion</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">*a</span></tt></td>
<td><tt class="literal"><span class="pre">T&</span></tt></td>
<td><tt class="literal"><span class="pre">T</span></tt> is <em>cv</em>
<tt class="literal"><span class="pre">iterator_traits<X>::value_type</span></tt>
where <em>cv</em> is an optional
cv-qualification.
pre: <tt class="literal"><span class="pre">a</span></tt> is
dereferenceable. If <tt class="literal"><span class="pre">a</span>
<span class="pre">==</span> <span class="pre">b</span></tt> then <tt class="literal"><span class="pre">*a</span></tt> is
equivalent to <tt class="literal"><span class="pre">*b</span></tt>.</td>
</tr>
</tbody>
</table>
</blockquote>
<p class="last">At the end of the section reverse_iterator models, add:
The type <tt class="literal"><span class="pre">iterator_traits<Iterator>::reference</span></tt> must be the type of
<tt class="literal"><span class="pre">*i</span></tt>, where <tt class="literal"><span class="pre">i</span></tt> is an object of type <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body"><p class="first">Ideally there should be requirements on the reference
type, however, since Readable Iterator is suppose to correspond
to the current standard iterator requirements (which do not place
requirements on the reference type) we will leave them off for
now. There is a DR in process with respect to the reference type
in the stadard iterator requirements. Once that is resolved we
will revisit this issue for Readable Iterator and Lvalue
Iterator.</p>
<p class="last">We added Assignable to the requirements for Readable
Iterator. This is needed to have Readable Iterator coincide with
the capabilities of Input Iterator.</p>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-facade-free-functions-unspecified">
<h2><a class="toc-backref" href="#id35" name="iterator-facade-free-functions-unspecified">9.34 iterator_facade free functions unspecified</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12562:</p>
<blockquote>
The template functions <tt class="literal"><span class="pre">operator==</span></tt>, <tt class="literal"><span class="pre">operator!=</span></tt>,
<tt class="literal"><span class="pre">operator<</span></tt>, <tt class="literal"><span class="pre">operator<=</span></tt>, <tt class="literal"><span class="pre">operator></span></tt>, <tt class="literal"><span class="pre">operator>=</span></tt>, and
<tt class="literal"><span class="pre">operator-</span></tt> that take two arguments that are specializations of
iterator_facade have no specification. The template function
operator+ that takes an argument that is a specialization of
iterator_facade and an argument of type difference_type has no
specification.</blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Add the missing specifications.</p>
<pre class="literal-block">
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
typename Derived::difference_type n);
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (typename Derived::difference_type n,
iterator_facade<Dr,V,TC,R,D> const&);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast<Derived const*>(this));
return tmp += n;
</pre>
</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt>, then
<tt class="literal"><span class="pre">lhs.equal(rhs)</span></tt>. Otherwise, <tt class="literal"><span class="pre">rhs.equal(lhs)</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt>, then
<tt class="literal"><span class="pre">!lhs.equal(rhs)</span></tt>. Otherwise, <tt class="literal"><span class="pre">!rhs.equal(lhs)</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt>, then
<tt class="literal"><span class="pre">lhs.distance_to(rhs)</span> <span class="pre"><</span> <span class="pre">0</span></tt>. Otherwise, <tt class="literal"><span class="pre">rhs.distance_to(lhs)</span> <span class="pre">></span>
<span class="pre">0</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt>, then
<tt class="literal"><span class="pre">lhs.distance_to(rhs)</span> <span class="pre"><=</span> <span class="pre">0</span></tt>. Otherwise, <tt class="literal"><span class="pre">rhs.distance_to(lhs)</span>
<span class="pre">>=</span> <span class="pre">0</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt>, then
<tt class="literal"><span class="pre">lhs.distance_to(rhs)</span> <span class="pre">></span> <span class="pre">0</span></tt>. Otherwise,
<tt class="literal"><span class="pre">rhs.distance_to(lhs)</span> <span class="pre"><</span> <span class="pre">0</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt>, then
<tt class="literal"><span class="pre">lhs.distance_to(rhs)</span> <span class="pre">>=</span> <span class="pre">0</span></tt>. Otherwise,
<tt class="literal"><span class="pre">rhs.distance_to(lhs)</span> <span class="pre"><=</span> <span class="pre">0</span></tt>.</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,difference>::type
operator -(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return Type:</th><td class="field-body">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt>, then
<tt class="literal"><span class="pre">difference</span></tt> shall be
<tt class="literal"><span class="pre">iterator_traits<Dr1>::difference_type</span></tt>. Otherwise,
<tt class="literal"><span class="pre">difference</span></tt> shall be
<tt class="literal"><span class="pre">iterator_traits<Dr2>::difference_type</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt>, then
<tt class="literal"><span class="pre">-lhs.distance_to(rhs)</span></tt>. Otherwise,
<tt class="literal"><span class="pre">rhs.distance_to(lhs)</span></tt>.</td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-facade-too-many-equals">
<h2><a class="toc-backref" href="#id36" name="iterator-facade-too-many-equals">9.35 iterator_facade: too many equals?</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12563:</p>
<blockquote>
<p>The table listing the functions required for types derived from
iterator_facade has two functions named equal and two named
distance_to:</p>
<pre class="literal-block">
c.equal(b)
c.equal(y)
c.distance_to(b)
c.distance_to(z)
</pre>
<p>where b and c are const objects of the derived type, y and z are
constant objects of certain iterator types that are interoperable
with the derived type. Seems like the 'b' versions are
redundant: in both cases, the other version will take a 'b'. In
fact, iterator_adaptor is specified to use iterator_facade, but
does not provide the 'b' versions of these functions.</p>
<p>Are the 'b' versions needed?</p>
</blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Remove the 'b' versions.</p>
<p>In <tt class="literal"><span class="pre">iterator_facade</span></tt> requirements, remove:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="19%" />
<col width="18%" />
<col width="36%" />
<col width="26%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.equal(b)</span></tt></td>
<td>convertible to bool</td>
<td>true iff <tt class="literal"><span class="pre">b</span></tt> and <tt class="literal"><span class="pre">c</span></tt> are
equivalent.</td>
<td>Single Pass Iterator</td>
</tr>
</tbody>
</table>
</blockquote>
<p>and remove:</p>
<blockquote class="last">
<table border class="table">
<colgroup>
<col width="19%" />
<col width="18%" />
<col width="36%" />
<col width="26%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.distance_to(b)</span></tt></td>
<td>convertible to
X::difference_type</td>
<td>equivalent to <tt class="literal"><span class="pre">distance(c,</span> <span class="pre">b)</span></tt></td>
<td>Random Access Traversal
Iterator</td>
</tr>
</tbody>
</table>
</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="iterator-facade-function-requirements">
<h2><a class="toc-backref" href="#id37" name="iterator-facade-function-requirements">9.36 iterator_facade function requirements</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12636:</p>
<blockquote>
<p>The table that lists required functions for the derived type X
passed to iterator_facade lists, among others:</p>
<p>for a single pass iterator:</p>
<pre class="literal-block">
c.equal(b)
c.equal(y)
</pre>
<p>where b and c are const X objects, and y is a const object of a
single pass iterator that is interoperable with X. Since X is
interoperable with itself, c.equal(b) is redundant. There is a
difference in their descriptions, but its meaning isn't
clear. The first is "true iff b and c are equivalent", and the
second is "true iff c and y refer to the same position." Is there
a difference between the undefined term "equivalent" and "refer
to the same position"?</p>
<p>Similarly, for a random access traversal iterator:</p>
<pre class="literal-block">
c.distance_to(b)
c.distance_to(z)
</pre>
<p>where z is a constant object of a random access traversal
iterator that is interoperable with X. Again, X is interoperable
with itself, so c.distance_to(b) is redundant. Also, the
specification for c.distance_to(z) isn't valid. It's written
as "equivalent to distance(c, z)". The template function distance
takes two arguments of the same type, so distance(c, z) isn't
valid if c and z are different types. Should it be
distance(c, (X)z)?</p>
</blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Removed the 'b' versions (see 9.35) and added the cast.</p>
<p>Change:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="19%" />
<col width="18%" />
<col width="36%" />
<col width="26%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.distance_to(z)</span></tt></td>
<td>convertible to
X::difference_type</td>
<td>equivalent to <tt class="literal"><span class="pre">distance(c,</span> <span class="pre">z)</span></tt>.
Implements <tt class="literal"><span class="pre">c</span> <span class="pre">-</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span> <span class="pre"><</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span>
<span class="pre"><=</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span> <span class="pre">></span> <span class="pre">z</span></tt>, and <tt class="literal"><span class="pre">c</span> <span class="pre">>=</span> <span class="pre">c</span></tt>.</td>
<td>Random Access Traversal
Iterator</td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote class="last">
<table border class="table">
<colgroup>
<col width="21%" />
<col width="23%" />
<col width="27%" />
<col width="29%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.distance_to(z)</span></tt></td>
<td>convertible to
<tt class="literal"><span class="pre">F::difference_type</span></tt></td>
<td>equivalent to
<tt class="literal"><span class="pre">distance(c,</span> <span class="pre">X(z))</span></tt>.</td>
<td>Random Access Traversal
Iterator</td>
</tr>
</tbody>
</table>
</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="more-issues-not-from-matt-s-list">
<h1><a class="toc-backref" href="#id38" name="more-issues-not-from-matt-s-list">More Issues (not from Matt's list)</a></h1>
<div class="section" id="x-inheritance-in-iterator-adaptor-and-other-adaptors-is-an-overspecification">
<h2><a class="toc-backref" href="#id39" name="x-inheritance-in-iterator-adaptor-and-other-adaptors-is-an-overspecification">9.37x Inheritance in iterator_adaptor and other adaptors is an overspecification</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12696:
The paper requires that iterator_adaptor be derived from an
appropriate instance of iterator_facade, and that most of the specific
forms of adaptors be derived from appropriate instances of
iterator_adaptor. That seems like overspecification, and we ought to
look at specifying these things in terms of what the various templates
provide rather than how they're implemented.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Remove the specfication of inheritance, and add explicit
specification of all the functionality that was inherited from the
specialized iterators.</p>
<p>In iterator_adaptor, inheritance is retained, sorry NAD. Also,
the Interoperable Iterators concept is added to the new iterator
concepts, and this concept is used in the specification of the
iterator adaptors.</p>
<p>In n1550, after [lib.random.access.traversal.iterators], add:</p>
<blockquote>
<p>Interoperable Iterators [lib.interoperable.iterators]</p>
<p>A class or built-in type <tt class="literal"><span class="pre">X</span></tt> that models Single Pass Iterator
is <em>interoperable with</em> a class or built-in type <tt class="literal"><span class="pre">Y</span></tt> that
also models Single Pass Iterator if the following expressions
are valid and respect the stated semantics. In the tables
below, <tt class="literal"><span class="pre">x</span></tt> is an object of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">y</span></tt> is an object of
type <tt class="literal"><span class="pre">Y</span></tt>, <tt class="literal"><span class="pre">Distance</span></tt> is
<tt class="literal"><span class="pre">iterator_traits<Y>::difference_type</span></tt>, and <tt class="literal"><span class="pre">n</span></tt> represents a
constant object of type <tt class="literal"><span class="pre">Distance</span></tt>.</p>
<table border class="table">
<colgroup>
<col width="13%" />
<col width="27%" />
<col width="60%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Precondition/Postcondition</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">y</span> <span class="pre">=</span> <span class="pre">x</span></tt></td>
<td><tt class="literal"><span class="pre">Y</span></tt></td>
<td>post: <tt class="literal"><span class="pre">y</span> <span class="pre">==</span> <span class="pre">x</span></tt></td>
</tr>
<tr><td><tt class="literal"><span class="pre">Y(x)</span></tt></td>
<td><tt class="literal"><span class="pre">Y</span></tt></td>
<td>post: <tt class="literal"><span class="pre">Y(x)</span> <span class="pre">==</span> <span class="pre">x</span></tt></td>
</tr>
<tr><td><tt class="literal"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">==</span></tt> is an equivalence relation over its domain.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">y</span> <span class="pre">==</span> <span class="pre">x</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">==</span></tt> is an equivalence relation over its domain.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">x</span> <span class="pre">!=</span> <span class="pre">y</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">bool(a==b)</span> <span class="pre">!=</span> <span class="pre">bool(a!=b)</span></tt> over its domain.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">y</span> <span class="pre">!=</span> <span class="pre">x</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">bool(a==b)</span> <span class="pre">!=</span> <span class="pre">bool(a!=b)</span></tt> over its domain.</td>
</tr>
</tbody>
</table>
<p>If <tt class="literal"><span class="pre">X</span></tt> and <tt class="literal"><span class="pre">Y</span></tt> both model Random Access Traversal Iterator then
the following additional requirements must be met.</p>
<table border class="table">
<colgroup>
<col width="12%" />
<col width="25%" />
<col width="23%" />
<col width="41%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Operational Semantics</th>
<th>Assertion/ Precondition</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">y</span> <span class="pre">-</span> <span class="pre">x</span> <span class="pre">></span> <span class="pre">0</span></tt></td>
<td><tt class="literal"><span class="pre"><</span></tt> is a total ordering relation</td>
</tr>
<tr><td><tt class="literal"><span class="pre">y</span> <span class="pre"><</span> <span class="pre">x</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">x</span> <span class="pre">-</span> <span class="pre">y</span> <span class="pre">></span> <span class="pre">0</span></tt></td>
<td><tt class="literal"><span class="pre"><</span></tt> is a total ordering relation</td>
</tr>
<tr><td><tt class="literal"><span class="pre">x</span> <span class="pre">></span> <span class="pre">y</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">y</span> <span class="pre"><</span> <span class="pre">x</span></tt></td>
<td><tt class="literal"><span class="pre">></span></tt> is a total ordering relation</td>
</tr>
<tr><td><tt class="literal"><span class="pre">y</span> <span class="pre">></span> <span class="pre">x</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">x</span> <span class="pre"><</span> <span class="pre">y</span></tt></td>
<td><tt class="literal"><span class="pre">></span></tt> is a total ordering relation</td>
</tr>
<tr><td><tt class="literal"><span class="pre">x</span> <span class="pre">>=</span> <span class="pre">y</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">!(x</span> <span class="pre"><</span> <span class="pre">y)</span></tt></td>
<td> </td>
</tr>
<tr><td><tt class="literal"><span class="pre">y</span> <span class="pre">>=</span> <span class="pre">x</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">!(y</span> <span class="pre"><</span> <span class="pre">x)</span></tt></td>
<td> </td>
</tr>
<tr><td><tt class="literal"><span class="pre">x</span> <span class="pre"><=</span> <span class="pre">y</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">!(x</span> <span class="pre">></span> <span class="pre">y)</span></tt></td>
<td> </td>
</tr>
<tr><td><tt class="literal"><span class="pre">y</span> <span class="pre"><=</span> <span class="pre">x</span></tt></td>
<td>convertible to <tt class="literal"><span class="pre">bool</span></tt></td>
<td><tt class="literal"><span class="pre">!(y</span> <span class="pre">></span> <span class="pre">x)</span></tt></td>
<td> </td>
</tr>
<tr><td><tt class="literal"><span class="pre">y</span> <span class="pre">-</span> <span class="pre">x</span></tt></td>
<td><tt class="literal"><span class="pre">Distance</span></tt></td>
<td><tt class="literal"><span class="pre">distance(Y(x),y)</span></tt></td>
<td>pre: there exists a value <tt class="literal"><span class="pre">n</span></tt> of
<tt class="literal"><span class="pre">Distance</span></tt> such that <tt class="literal"><span class="pre">x</span> <span class="pre">+</span> <span class="pre">n</span> <span class="pre">==</span> <span class="pre">y</span></tt>.
<tt class="literal"><span class="pre">y</span> <span class="pre">==</span> <span class="pre">x</span> <span class="pre">+</span> <span class="pre">(y</span> <span class="pre">-</span> <span class="pre">x)</span></tt>.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">x</span> <span class="pre">-</span> <span class="pre">y</span></tt></td>
<td><tt class="literal"><span class="pre">Distance</span></tt></td>
<td><tt class="literal"><span class="pre">distance(y,Y(x))</span></tt></td>
<td>pre: there exists a value <tt class="literal"><span class="pre">n</span></tt> of
<tt class="literal"><span class="pre">Distance</span></tt> such that <tt class="literal"><span class="pre">y</span> <span class="pre">+</span> <span class="pre">n</span> <span class="pre">==</span> <span class="pre">x</span></tt>.
<tt class="literal"><span class="pre">x</span> <span class="pre">==</span> <span class="pre">y</span> <span class="pre">+</span> <span class="pre">(x</span> <span class="pre">-</span> <span class="pre">y)</span></tt>.</td>
</tr>
</tbody>
</table>
</blockquote>
<p>In N1530:</p>
<blockquote class="last">
<p>In [lib.iterator.adaptor]</p>
<p>Change:</p>
<pre class="literal-block">
class iterator_adaptor
: public iterator_facade<Derived, /* see details ...*/>
</pre>
<p>To:</p>
<pre class="literal-block">
class iterator_adaptor
: public iterator_facade<Derived, *V'*, *C'*, *R'*, *D'*> // see details
</pre>
<dl>
<dt>Change the text from:</dt>
<dd>The <tt class="literal"><span class="pre">Base</span></tt> type must implement the expressions involving
<tt class="literal"><span class="pre">m_iterator</span></tt> in the specifications...</dd>
<dt>until the end of the <strong>iterator_adaptor requirements</strong> section, to:</dt>
<dd>The <tt class="literal"><span class="pre">Base</span></tt> argument shall be Assignable and Copy Constructible.</dd>
</dl>
<p>Add:</p>
</blockquote>
</td>
</tr>
</tbody>
</table>
<div class="topic">
<p class="topic-title"><tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class parameters</p>
<blockquote>
<p>The <em>V'</em>, <em>C'</em>, <em>R'</em>, and <em>D'</em> parameters of the <tt class="literal"><span class="pre">iterator_facade</span></tt>
used as a base class in the summary of <tt class="literal"><span class="pre">iterator_adaptor</span></tt>
above are defined as follows:</p>
<pre class="literal-block">
<em>V'</em> = if (Value is use_default)
return iterator_traits<Base>::value_type
else
return Value
<em>C'</em> = if (CategoryOrTraversal is use_default)
return iterator_traversal<Base>::type
else
return CategoryOrTraversal
<em>R'</em> = if (Reference is use_default)
if (Value is use_default)
return iterator_traits<Base>::reference
else
return Value&
else
return Reference
<em>D'</em> = if (Difference is use_default)
return iterator_traits<Base>::difference_type
else
return Difference
</pre>
</blockquote>
<p>In [lib.iterator.special.adaptors]</p>
<p>Change:</p>
<pre class="literal-block">
class indirect_iterator
: public iterator_adaptor</* see discussion */>
{
friend class iterator_core_access;
</pre>
<p>to:</p>
<pre class="literal-block">
class indirect_iterator
{
public:
typedef /* see below */ value_type;
typedef /* see below */ reference;
typedef /* see below */ pointer;
typedef /* see below */ difference_type;
typedef /* see below */ iterator_category;
</pre>
<p>Change:</p>
<pre class="literal-block">
private: // as-if specification
typename indirect_iterator::reference dereference() const
{
return **this->base();
}
</pre>
<p>to:</p>
<pre class="literal-block">
Iterator const& base() const;
reference operator*() const;
indirect_iterator& operator++();
indirect_iterator& operator--();
private:
Iterator m_iterator; // exposition
</pre>
<p>After the synopsis add:</p>
<blockquote>
<p>The member types of <tt class="literal"><span class="pre">indirect_iterator</span></tt> are defined
according to the following pseudo-code, where <tt class="literal"><span class="pre">V</span></tt> is
<tt class="literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt></p>
<pre class="literal-block">
if (Value is use_default) then
typedef remove_const<pointee<V>::type>::type value_type;
else
typedef remove_const<Value>::type value_type;
if (Reference is use_default) then
if (Value is use_default) then
typedef indirect_reference<V>::type reference;
else
typedef Value& reference;
else
typedef Reference reference;
if (Value is use_default) then
typedef pointee<V>::type* pointer;
else
typedef Value* pointer;
if (Difference is use_default)
typedef iterator_traits<Iterator>::difference_type difference_type;
else
typedef Difference difference_type;
if (CategoryOrTraversal is use_default)
typedef <em>iterator-category</em>(
iterator_traversal<Iterator>::type,``reference``,``value_type``
) iterator_category;
else
typedef <em>iterator-category</em>(
CategoryOrTraversal,``reference``,``value_type``
) iterator_category;
</pre>
</blockquote>
<p>[Note: See resolution to 9.44y for a description of <tt class="literal"><span class="pre">pointee</span></tt> and
<tt class="literal"><span class="pre">indirect_reference</span></tt>]</p>
<p>After the requirements section, add:</p>
</div>
<div class="topic">
<p class="topic-title"><tt class="literal"><span class="pre">indirect_iterator</span></tt> models</p>
<blockquote>
<p>In addition to the concepts indicated by <tt class="literal"><span class="pre">iterator_category</span></tt>
and by <tt class="literal"><span class="pre">iterator_traversal<indirect_iterator>::type</span></tt>, a
specialization of <tt class="literal"><span class="pre">indirect_iterator</span></tt> models the following
concepts, Where <tt class="literal"><span class="pre">v</span></tt> is an object of
<tt class="literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt>:</p>
<blockquote>
<ul class="simple">
<li>Readable Iterator if <tt class="literal"><span class="pre">reference(*v)</span></tt> is convertible to
<tt class="literal"><span class="pre">value_type</span></tt>.</li>
<li>Writable Iterator if <tt class="literal"><span class="pre">reference(*v)</span> <span class="pre">=</span> <span class="pre">t</span></tt> is a valid
expression (where <tt class="literal"><span class="pre">t</span></tt> is an object of type
<tt class="literal"><span class="pre">indirect_iterator::value_type</span></tt>)</li>
<li>Lvalue Iterator if <tt class="literal"><span class="pre">reference</span></tt> is a reference type.</li>
</ul>
</blockquote>
<p><tt class="literal"><span class="pre">indirect_iterator<X,V1,C1,R1,D1></span></tt> is interoperable with
<tt class="literal"><span class="pre">indirect_iterator<Y,V2,C2,R2,D2></span></tt> if and only if <tt class="literal"><span class="pre">X</span></tt> is
interoperable with <tt class="literal"><span class="pre">Y</span></tt>.</p>
</blockquote>
<p>Before <tt class="literal"><span class="pre">indirect_iterator();</span></tt> add:</p>
<blockquote>
In addition to the operations required by the concepts described
above, specializations of <tt class="literal"><span class="pre">indirect_iterator</span></tt> provide the
following operations.</blockquote>
<dl>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> with
the <tt class="literal"><span class="pre">iterator_adaptor</span></tt> subobject copy constructed from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">indirect_iterator</span></tt> with
<tt class="literal"><span class="pre">m_iterator</span></tt> copy constructed from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
</dl>
<p>At the end of the indirect_iterator operations add:</p>
<blockquote>
<p><tt class="literal"><span class="pre">Iterator</span> <span class="pre">const&</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_iterator</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">**m_iterator</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">indirect_iterator&</span> <span class="pre">operator++();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">++m_iterator</span></tt></td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">indirect_iterator&</span> <span class="pre">operator--();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">--m_iterator</span></tt></td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
<p>Change:</p>
<pre class="literal-block">
template <class Iterator>
class reverse_iterator :
public iterator_adaptor< reverse_iterator<Iterator>, Iterator >
{
friend class iterator_core_access;
</pre>
<p>to:</p>
<pre class="literal-block">
template <class Iterator>
class reverse_iterator
{
public:
typedef iterator_traits<Iterator>::value_type value_type;
typedef iterator_traits<Iterator>::reference reference;
typedef iterator_traits<Iterator>::pointer pointer;
typedef iterator_traits<Iterator>::difference_type difference_type;
typedef /* see below */ iterator_category;
</pre>
<p>Change:</p>
<pre class="literal-block">
private: // as-if specification
typename reverse_iterator::reference dereference() const { return *prior(this->base()); }
void increment() { --this->base_reference(); }
void decrement() { ++this->base_reference(); }
void advance(typename reverse_iterator::difference_type n)
{
this->base_reference() += -n;
}
template <class OtherIterator>
typename reverse_iterator::difference_type
distance_to(reverse_iterator<OtherIterator> const& y) const
{
return this->base_reference() - y.base();
}
</pre>
<p>to:</p>
<pre class="literal-block">
Iterator const& base() const;
reference operator*() const;
reverse_iterator& operator++();
reverse_iterator& operator--();
private:
Iterator m_iterator; // exposition
</pre>
<dl>
<dt>After the synopsis for <tt class="literal"><span class="pre">reverse_iterator</span></tt>, add:</dt>
<dd>If <tt class="literal"><span class="pre">Iterator</span></tt> models Random Access Traversal Iterator and Readable
Lvalue Iterator, then <tt class="literal"><span class="pre">iterator_category</span></tt> is convertible to
<tt class="literal"><span class="pre">random_access_iterator_tag</span></tt>. Otherwise, if
<tt class="literal"><span class="pre">Iterator</span></tt> models Bidirectional Traversal Iterator and Readable
Lvalue Iterator, then <tt class="literal"><span class="pre">iterator_category</span></tt> is convertible to
<tt class="literal"><span class="pre">bidirectional_iterator_tag</span></tt>. Otherwise, <tt class="literal"><span class="pre">iterator_category</span></tt> is
convertible to <tt class="literal"><span class="pre">input_iterator_tag</span></tt>.</dd>
<dt>Change:</dt>
<dd><p class="first"><strong>reverse_iterator requirements</strong></p>
<p class="last">The base <tt class="literal"><span class="pre">Iterator</span></tt> must be a model of Bidirectional Traversal
Iterator. The resulting <tt class="literal"><span class="pre">reverse_iterator</span></tt> will be a model of the
most refined standard traversal and access concepts that are modeled
by <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
</dd>
<dt>to:</dt>
<dd><p class="first"><strong>reverse_iterator requirements</strong></p>
<p class="last"><tt class="literal"><span class="pre">Iterator</span></tt> must be a model of Bidirectional Traversal Iterator.</p>
</dd>
</dl>
</div>
<div class="topic">
<p class="topic-title"><tt class="literal"><span class="pre">reverse_iterator</span></tt> models</p>
<blockquote>
<p>A specialization of <tt class="literal"><span class="pre">reverse_iterator</span></tt> models the same iterator
traversal and iterator access concepts modeled by its <tt class="literal"><span class="pre">Iterator</span></tt>
argument. In addition, it may model old iterator concepts
specified in the following table:</p>
<table border class="table">
<colgroup>
<col width="53%" />
<col width="47%" />
</colgroup>
<thead valign="bottom">
<tr><th>If <tt class="literal"><span class="pre">I</span></tt> models</th>
<th>then <tt class="literal"><span class="pre">reverse_iterator<I></span></tt> models</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>Readable Lvalue Iterator,
Bidirectional Traversal Iterator</td>
<td>Bidirectional Iterator</td>
</tr>
<tr><td>Writable Lvalue Iterator,
Bidirectional Traversal Iterator</td>
<td>Mutable Bidirectional Iterator</td>
</tr>
<tr><td>Readable Lvalue Iterator,
Random Access Traversal Iterator</td>
<td>Random Access Iterator</td>
</tr>
<tr><td>Writable Lvalue Iterator,
Random Access Traversal Iterator</td>
<td>Mutable Random Access Iterator</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">reverse_iterator<X></span></tt> is interoperable with
<tt class="literal"><span class="pre">reverse_iterator<Y></span></tt> if and only if <tt class="literal"><span class="pre">X</span></tt> is interoperable with
<tt class="literal"><span class="pre">Y</span></tt>.</p>
</blockquote>
<dl>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> with a
default constructed base object.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> with <tt class="literal"><span class="pre">m_iterator</span></tt>
default constructed.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> with a
base object copy constructed from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs an instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> with a
<tt class="literal"><span class="pre">m_iterator</span></tt> constructed from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> that is a copy of <tt class="literal"><span class="pre">r</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs instance of <tt class="literal"><span class="pre">reverse_iterator</span></tt> whose
<tt class="literal"><span class="pre">m_iterator</span></tt> subobject is constructed from <tt class="literal"><span class="pre">y.base()</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>At the end of the operations for <tt class="literal"><span class="pre">reverse_iterator</span></tt>, add:</dt>
<dd><p class="first"><tt class="literal"><span class="pre">Iterator</span> <span class="pre">const&</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_iterator</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"></td>
</tr>
</tbody>
</table>
<pre class="literal-block">
Iterator tmp = m_iterator;
return *--tmp;
</pre>
<p><tt class="literal"><span class="pre">reverse_iterator&</span> <span class="pre">operator++();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">--m_iterator</span></tt></td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">reverse_iterator&</span> <span class="pre">operator--();</span></tt></p>
<table class="last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">++m_iterator</span></tt></td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
</dd>
</dl>
<p>Change:</p>
<pre class="literal-block">
class transform_iterator
: public iterator_adaptor</* see discussion */>
{
friend class iterator_core_access;
</pre>
<p>to:</p>
<pre class="literal-block">
class transform_iterator
{
public:
typedef /* see below */ value_type;
typedef /* see below */ reference;
typedef /* see below */ pointer;
typedef iterator_traits<Iterator>::difference_type difference_type;
typedef /* see below */ iterator_category;
</pre>
<p>After <tt class="literal"><span class="pre">UnaryFunction</span> <span class="pre">functor()</span> <span class="pre">const;</span></tt> add:</p>
<pre class="literal-block">
Iterator const& base() const;
reference operator*() const;
transform_iterator& operator++();
transform_iterator& operator--();
</pre>
<p>Change:</p>
<pre class="literal-block">
private:
typename transform_iterator::value_type dereference() const;
UnaryFunction m_f;
};
</pre>
<p>to:</p>
<pre class="literal-block">
private:
Iterator m_iterator; // exposition only
UnaryFunction m_f; // exposition only
};
</pre>
<dl>
<dt>After the synopsis, add:</dt>
<dd>If <tt class="literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and if <tt class="literal"><span class="pre">Iterator</span></tt>
models Random Access Traversal Iterator, then <tt class="literal"><span class="pre">iterator_category</span></tt> is
convertible to <tt class="literal"><span class="pre">random_access_iterator_tag</span></tt>. Otherwise, if
<tt class="literal"><span class="pre">Iterator</span></tt> models Bidirectional Traversal Iterator, then
<tt class="literal"><span class="pre">iterator_category</span></tt> is convertible to
<tt class="literal"><span class="pre">bidirectional_iterator_tag</span></tt>. Otherwise <tt class="literal"><span class="pre">iterator_category</span></tt> is
convertible to <tt class="literal"><span class="pre">forward_iterator_tag</span></tt>. If <tt class="literal"><span class="pre">Iterator</span></tt> does not
model Readable Lvalue Iterator then <tt class="literal"><span class="pre">iterator_category</span></tt> is
convertible to <tt class="literal"><span class="pre">input_iterator_tag</span></tt>.</dd>
<dt>In the requirements section, change:</dt>
<dd><p class="first">The type <tt class="literal"><span class="pre">Iterator</span></tt> must at least model Readable Iterator. The
resulting <tt class="literal"><span class="pre">transform_iterator</span></tt> models the most refined of the
following that is also modeled by <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<blockquote>
<ul class="simple">
<li>Writable Lvalue Iterator if
<tt class="literal"><span class="pre">result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type</span></tt>
is a non-const reference.</li>
<li>Readable Lvalue Iterator if
<tt class="literal"><span class="pre">result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type</span></tt>
is a const reference.</li>
<li>Readable Iterator otherwise.</li>
</ul>
</blockquote>
<p>The <tt class="literal"><span class="pre">transform_iterator</span></tt> models the most refined standard traversal
concept that is modeled by <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<p class="last">The <tt class="literal"><span class="pre">reference</span></tt> type of <tt class="literal"><span class="pre">transform_iterator</span></tt> is
<tt class="literal"><span class="pre">result_of<UnaryFunction(iterator_traits<Iterator>::reference)>::type</span></tt>.
The <tt class="literal"><span class="pre">value_type</span></tt> is <tt class="literal"><span class="pre">remove_cv<remove_reference<reference></span> <span class="pre">>::type</span></tt>.</p>
</dd>
<dt>to:</dt>
<dd>The argument <tt class="literal"><span class="pre">Iterator</span></tt> shall model Readable Iterator.</dd>
</dl>
<p>After the requirements section, add:</p>
</div>
<div class="topic">
<p class="topic-title"><tt class="literal"><span class="pre">transform_iterator</span></tt> models</p>
<blockquote>
<p>The resulting <tt class="literal"><span class="pre">transform_iterator</span></tt> models the most refined of the
following options that is also modeled by <tt class="literal"><span class="pre">Iterator</span></tt>.</p>
<blockquote>
<ul class="simple">
<li>Writable Lvalue Iterator if
<tt class="literal"><span class="pre">transform_iterator::reference</span></tt> is a non-const
reference.</li>
<li>Readable Lvalue Iterator if
<tt class="literal"><span class="pre">transform_iterator::reference</span></tt> is a const reference.</li>
<li>Readable Iterator otherwise.</li>
</ul>
</blockquote>
<p>The <tt class="literal"><span class="pre">transform_iterator</span></tt> models the most refined standard traversal
concept that is modeled by the <tt class="literal"><span class="pre">Iterator</span></tt> argument.</p>
<p>If <tt class="literal"><span class="pre">transform_iterator</span></tt> is a model of Readable Lvalue Iterator then
it models the following original iterator concepts depending on what
the <tt class="literal"><span class="pre">Iterator</span></tt> argument models.</p>
<table border class="table">
<colgroup>
<col width="50%" />
<col width="50%" />
</colgroup>
<thead valign="bottom">
<tr><th>If <tt class="literal"><span class="pre">Iterator</span></tt> models</th>
<th>then <tt class="literal"><span class="pre">transform_iterator</span></tt> models</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>Single Pass Iterator</td>
<td>Input Iterator</td>
</tr>
<tr><td>Forward Traversal Iterator</td>
<td>Forward Iterator</td>
</tr>
<tr><td>Bidirectional Traversal Iterator</td>
<td>Bidirectional Iterator</td>
</tr>
<tr><td>Random Access Traversal Iterator</td>
<td>Random Access Iterator</td>
</tr>
</tbody>
</table>
<p>If <tt class="literal"><span class="pre">transform_iterator</span></tt> models Writable Lvalue Iterator then it is a
mutable iterator (as defined in the old iterator requirements).</p>
<p><tt class="literal"><span class="pre">transform_iterator<F1,</span> <span class="pre">X,</span> <span class="pre">R1,</span> <span class="pre">V1></span></tt> is interoperable with
<tt class="literal"><span class="pre">transform_iterator<F2,</span> <span class="pre">Y,</span> <span class="pre">R2,</span> <span class="pre">V2></span></tt> if and only if <tt class="literal"><span class="pre">X</span></tt> is
interoperable with <tt class="literal"><span class="pre">Y</span></tt>.</p>
</blockquote>
<p>Remove the private operations section heading and remove:</p>
<pre class="literal-block">
``typename transform_iterator::value_type dereference() const;``
:Returns: ``m_f(transform_iterator::dereference());``
</pre>
<p>After the entry for <tt class="literal"><span class="pre">functor()</span></tt>, add:</p>
<pre class="literal-block">
``Iterator const& base() const;``
:Returns: ``m_iterator``
``reference operator*() const;``
:Returns: ``m_f(*m_iterator)``
``transform_iterator& operator++();``
:Effects: ``++m_iterator``
:Returns: ``*this``
``transform_iterator& operator--();``
:Effects: ``--m_iterator``
:Returns: ``*this``
</pre>
<p>Change:</p>
<pre class="literal-block">
template <class Predicate, class Iterator>
class filter_iterator
: public iterator_adaptor<
filter_iterator<Predicate, Iterator>, Iterator
, use_default
, /* see details */
>
{
public:
</pre>
<p>to:</p>
<pre class="literal-block">
template <class Predicate, class Iterator>
class filter_iterator
{
public:
typedef iterator_traits<Iterator>::value_type value_type;
typedef iterator_traits<Iterator>::reference reference;
typedef iterator_traits<Iterator>::pointer pointer;
typedef iterator_traits<Iterator>::difference_type difference_type;
typedef /* see below */ iterator_category;
</pre>
<p>Change:</p>
<pre class="literal-block">
private: // as-if specification
void increment()
{
++(this->base_reference());
satisfy_predicate();
}
void satisfy_predicate()
{
while (this->base() != this->m_end && !this->m_predicate(*this->base()))
++(this->base_reference());
}
Predicate m_predicate;
Iterator m_end;
</pre>
<p>to:</p>
<pre class="literal-block">
Iterator const& base() const;
reference operator*() const;
filter_iterator& operator++();
private:
Predicate m_pred; // exposition only
Iterator m_iter; // exposition only
Iterator m_end; // exposition only
</pre>
<dl>
<dt>Change:</dt>
<dd>The base <tt class="literal"><span class="pre">Iterator</span></tt> parameter must be a model of Readable
Iterator and Single Pass Iterator. The resulting
<tt class="literal"><span class="pre">filter_iterator</span></tt> will be a model of Forward Traversal Iterator
if <tt class="literal"><span class="pre">Iterator</span></tt> is, otherwise the <tt class="literal"><span class="pre">filter_iterator</span></tt> will be a
model of Single Pass Iterator. The access category of the
<tt class="literal"><span class="pre">filter_iterator</span></tt> will be the same as the access category of
<tt class="literal"><span class="pre">Iterator</span></tt>.</dd>
<dt>to:</dt>
<dd>The <tt class="literal"><span class="pre">Iterator</span></tt> argument shall meet the requirements of Readable
Iterator and Single Pass Iterator or it shall meet the requirements of
Input Iterator.</dd>
</dl>
<p>After the requirements section, add:</p>
</div>
<div class="topic">
<p class="topic-title"><tt class="literal"><span class="pre">filter_iterator</span></tt> models</p>
<blockquote>
<p>The concepts that <tt class="literal"><span class="pre">filter_iterator</span></tt> models are dependent on which
concepts the <tt class="literal"><span class="pre">Iterator</span></tt> argument models, as specified in the
following tables.</p>
<table border class="table">
<colgroup>
<col width="33%" />
<col width="67%" />
</colgroup>
<thead valign="bottom">
<tr><th>If <tt class="literal"><span class="pre">Iterator</span></tt> models</th>
<th>then <tt class="literal"><span class="pre">filter_iterator</span></tt> models</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>Single Pass Iterator</td>
<td>Single Pass Iterator</td>
</tr>
<tr><td>Forward Traversal Iterator</td>
<td>Forward Traversal Iterator</td>
</tr>
</tbody>
</table>
<table border class="table">
<colgroup>
<col width="41%" />
<col width="59%" />
</colgroup>
<thead valign="bottom">
<tr><th>If <tt class="literal"><span class="pre">Iterator</span></tt> models</th>
<th>then <tt class="literal"><span class="pre">filter_iterator</span></tt> models</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>Readable Iterator</td>
<td>Readable Iterator</td>
</tr>
<tr><td>Writable Iterator</td>
<td>Writable Iterator</td>
</tr>
<tr><td>Lvalue Iterator</td>
<td>Lvalue Iterator</td>
</tr>
</tbody>
</table>
<table border class="table">
<colgroup>
<col width="63%" />
<col width="38%" />
</colgroup>
<thead valign="bottom">
<tr><th>If <tt class="literal"><span class="pre">Iterator</span></tt> models</th>
<th>then <tt class="literal"><span class="pre">filter_iterator</span></tt> models</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>Readable Iterator, Single Pass Iterator</td>
<td>Input Iterator</td>
</tr>
<tr><td>Readable Lvalue Iterator, Forward Traversal Iterator</td>
<td>Forward Iterator</td>
</tr>
<tr><td>Writable Lvalue Iterator, Forward Traversal Iterator</td>
<td>Mutable Forward Iterator</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">filter_iterator<P1,</span> <span class="pre">X></span></tt> is interoperable with <tt class="literal"><span class="pre">filter_iterator<P2,</span> <span class="pre">Y></span></tt>
if and only if <tt class="literal"><span class="pre">X</span></tt> is interoperable with <tt class="literal"><span class="pre">Y</span></tt>.</p>
</blockquote>
<dl>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">a <tt class="literal"><span class="pre">filter_iterator</span></tt> whose
predicate is a default constructed <tt class="literal"><span class="pre">Predicate</span></tt> and
whose <tt class="literal"><span class="pre">end</span></tt> is a default constructed <tt class="literal"><span class="pre">Iterator</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="literal"><span class="pre">filter_iterator</span></tt> whose``m_pred``, <tt class="literal"><span class="pre">m_iter</span></tt>, and <tt class="literal"><span class="pre">m_end</span></tt>
members are a default constructed.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A <tt class="literal"><span class="pre">filter_iterator</span></tt> at position <tt class="literal"><span class="pre">x</span></tt> that filters according
to predicate <tt class="literal"><span class="pre">f</span></tt> and that will not increment past <tt class="literal"><span class="pre">end</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="literal"><span class="pre">filter_iterator</span></tt> where <tt class="literal"><span class="pre">m_iter</span></tt> is either
the first position in the range <tt class="literal"><span class="pre">[x,end)</span></tt> such that <tt class="literal"><span class="pre">f(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>
or else``m_iter == end``. The member <tt class="literal"><span class="pre">m_pred</span></tt> is constructed from
<tt class="literal"><span class="pre">f</span></tt> and <tt class="literal"><span class="pre">m_end</span></tt> from <tt class="literal"><span class="pre">end</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A <tt class="literal"><span class="pre">filter_iterator</span></tt> at position <tt class="literal"><span class="pre">x</span></tt> that filters
according to a default constructed <tt class="literal"><span class="pre">Predicate</span></tt>
and that will not increment past <tt class="literal"><span class="pre">end</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a <tt class="literal"><span class="pre">filter_iterator</span></tt> where <tt class="literal"><span class="pre">m_iter</span></tt> is either
the first position in the range <tt class="literal"><span class="pre">[x,end)</span></tt> such that <tt class="literal"><span class="pre">m_pred(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>
or else``m_iter == end``. The member <tt class="literal"><span class="pre">m_pred</span></tt> is default constructed.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A copy of iterator <tt class="literal"><span class="pre">t</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Constructs a filter iterator whose members are copied from <tt class="literal"><span class="pre">t</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A copy of the predicate object used to construct <tt class="literal"><span class="pre">*this</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_pred</span></tt></td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">The object <tt class="literal"><span class="pre">end</span></tt> used to construct <tt class="literal"><span class="pre">*this</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_end</span></tt></td>
</tr>
</tbody>
</table>
</dd>
</dl>
<p>At the end of the operations section, add:</p>
<blockquote>
<p><tt class="literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*m_iter</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">filter_iterator&</span> <span class="pre">operator++();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Increments <tt class="literal"><span class="pre">m_iter</span></tt> and then continues to
increment <tt class="literal"><span class="pre">m_iter</span></tt> until either <tt class="literal"><span class="pre">m_iter</span> <span class="pre">==</span> <span class="pre">m_end</span></tt>
or <tt class="literal"><span class="pre">m_pred(*m_iter)</span> <span class="pre">==</span> <span class="pre">true</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
<p>Change:</p>
<pre class="literal-block">
class counting_iterator
: public iterator_adaptor<
counting_iterator<Incrementable, Access, Traversal, Difference>
, Incrementable
, Incrementable
, Access
, /* see details for traversal category */
, Incrementable const&
, Incrementable const*
, /* distance = Difference or a signed integral type */>
{
friend class iterator_core_access;
public:
</pre>
<p>to:</p>
<pre class="literal-block">
class counting_iterator
{
public:
typedef Incrementable value_type;
typedef const Incrementable& reference;
typedef const Incrementable* pointer;
typedef /* see below */ difference_type;
typedef /* see below */ iterator_category;
</pre>
<p>Change:</p>
<pre class="literal-block">
private:
typename counting_iterator::reference dereference() const
{
return this->base_reference();
}
</pre>
<p>to:</p>
<pre class="literal-block">
Incrementable const& base() const;
reference operator*() const;
counting_iterator& operator++();
counting_iterator& operator--();
private:
Incrementable m_inc; // exposition
</pre>
<p>After the synopsis, add:</p>
<blockquote>
<p>If the <tt class="literal"><span class="pre">Difference</span></tt> argument is <tt class="literal"><span class="pre">use_default</span></tt> then
<tt class="literal"><span class="pre">difference_type</span></tt> is an unspecified signed integral
type. Otherwise <tt class="literal"><span class="pre">difference_type</span></tt> is <tt class="literal"><span class="pre">Difference</span></tt>.</p>
<p><tt class="literal"><span class="pre">iterator_category</span></tt> is determined according to the following
algorithm:</p>
<pre class="literal-block">
if (CategoryOrTraversal is not use_default)
return CategoryOrTraversal
else if (numeric_limits<Incrementable>::is_specialized)
return <em>iterator-category</em>(
random_access_traversal_tag, Incrementable, const Incrementable&)
else
return <em>iterator-category</em>(
iterator_traversal<Incrementable>::type,
Incrementable, const Incrementable&)
</pre>
</blockquote>
<dl>
<dt>Change:</dt>
<dd><dl class="first last">
<dt>[<em>Note:</em> implementers are encouraged to provide an implementation of</dt>
<dd><tt class="literal"><span class="pre">distance_to</span></tt> and a <tt class="literal"><span class="pre">difference_type</span></tt> that avoids overflows in
the cases when the <tt class="literal"><span class="pre">Incrementable</span></tt> type is a numeric type.]</dd>
</dl>
</dd>
<dt>to:</dt>
<dd><dl class="first last">
<dt>[<em>Note:</em> implementers are encouraged to provide an implementation of</dt>
<dd><tt class="literal"><span class="pre">operator-</span></tt> and a <tt class="literal"><span class="pre">difference_type</span></tt> that avoid overflows in
the cases where <tt class="literal"><span class="pre">std::numeric_limits<Incrementable>::is_specialized</span></tt>
is true.]</dd>
</dl>
</dd>
<dt>Change:</dt>
<dd><p class="first">The <tt class="literal"><span class="pre">Incrementable</span></tt> type must be Default Constructible, Copy
Constructible, and Assignable. The default distance is
an implementation defined signed integegral type.</p>
<p class="last">The resulting <tt class="literal"><span class="pre">counting_iterator</span></tt> models Readable Lvalue Iterator.</p>
</dd>
<dt>to:</dt>
<dd>The <tt class="literal"><span class="pre">Incrementable</span></tt> argument shall be Copy Constructible and Assignable.</dd>
<dt>Change:</dt>
<dd>Furthermore, if you wish to create a counting iterator that is a Forward
Traversal Iterator, then the following expressions must be valid:</dd>
<dt>to:</dt>
<dd>If <tt class="literal"><span class="pre">iterator_category</span></tt> is convertible to <tt class="literal"><span class="pre">forward_iterator_tag</span></tt>
or <tt class="literal"><span class="pre">forward_traversal_tag</span></tt>, the following must be well-formed:</dd>
<dt>Change:</dt>
<dd>If you wish to create a counting iterator that is a
Bidirectional Traversal Iterator, then pre-decrement is also required:</dd>
<dt>to:</dt>
<dd>If <tt class="literal"><span class="pre">iterator_category</span></tt> is convertible to
<tt class="literal"><span class="pre">bidirectional_iterator_tag</span></tt> or <tt class="literal"><span class="pre">bidirectional_traversal_tag</span></tt>,
the following expression must also be well-formed:</dd>
<dt>Change:</dt>
<dd>If you wish to create a counting iterator that is a Random Access
Traversal Iterator, then these additional expressions are also
required:</dd>
<dt>to:</dt>
<dd>If <tt class="literal"><span class="pre">iterator_category</span></tt> is convertible to
<tt class="literal"><span class="pre">random_access_iterator_tag</span></tt> or <tt class="literal"><span class="pre">random_access_traversal_tag</span></tt>,
the following must must also be valid:</dd>
</dl>
<p>After the requirements section, add:</p>
</div>
<div class="topic">
<p class="topic-title"><tt class="literal"><span class="pre">counting_iterator</span></tt> models</p>
<blockquote>
<p>Specializations of <tt class="literal"><span class="pre">counting_iterator</span></tt> model Readable Lvalue
Iterator. In addition, they model the concepts corresponding to the
iterator tags to which their <tt class="literal"><span class="pre">iterator_category</span></tt> is convertible.
Also, if <tt class="literal"><span class="pre">CategoryOrTraversal</span></tt> is not <tt class="literal"><span class="pre">use_default</span></tt> then
<tt class="literal"><span class="pre">counting_iterator</span></tt> models the concept corresponding to the iterator
tag <tt class="literal"><span class="pre">CategoryOrTraversal</span></tt>. Otherwise, if
<tt class="literal"><span class="pre">numeric_limits<Incrementable>::is_specialized</span></tt>, then
<tt class="literal"><span class="pre">counting_iterator</span></tt> models Random Access Traversal Iterator.
Otherwise, <tt class="literal"><span class="pre">counting_iterator</span></tt> models the same iterator traversal
concepts modeled by <tt class="literal"><span class="pre">Incrementable</span></tt>.</p>
<p><tt class="literal"><span class="pre">counting_iterator<X,C1,D1></span></tt> is interoperable with
<tt class="literal"><span class="pre">counting_iterator<Y,C2,D2></span></tt> if and only if <tt class="literal"><span class="pre">X</span></tt> is
interoperable with <tt class="literal"><span class="pre">Y</span></tt>.</p>
</blockquote>
<p>At the begining of the operations section, add:</p>
<blockquote>
In addition to the operations required by the concepts modeled by
<tt class="literal"><span class="pre">counting_iterator</span></tt>, <tt class="literal"><span class="pre">counting_iterator</span></tt> provides the following
operations.</blockquote>
<dl>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">A default constructed instance of <tt class="literal"><span class="pre">counting_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body"><tt class="literal"><span class="pre">Incrementable</span></tt> is Default Constructible.</td>
</tr>
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Default construct the member <tt class="literal"><span class="pre">m_inc</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">counting_iterator</span></tt> that is a copy of <tt class="literal"><span class="pre">rhs</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Construct member <tt class="literal"><span class="pre">m_inc</span></tt> from <tt class="literal"><span class="pre">rhs.m_inc</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>Change:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">An instance of <tt class="literal"><span class="pre">counting_iterator</span></tt> with its base
object copy constructed from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
<dt>to:</dt>
<dd><table class="first last field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body">Construct member <tt class="literal"><span class="pre">m_inc</span></tt> from <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
</dd>
</dl>
<p>At the end of the operations section, add:</p>
<blockquote>
<p><tt class="literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_inc</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">counting_iterator&</span> <span class="pre">operator++();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">++m_inc</span></tt></td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">counting_iterator&</span> <span class="pre">operator--();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><tt class="literal"><span class="pre">--m_inc</span></tt></td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">*this</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Incrementable</span> <span class="pre">const&</span> <span class="pre">base()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">m_inc</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
</div>
</div>
<div class="section" id="x-problem-with-specification-of-a-m-in-readable-iterator">
<h2><a class="toc-backref" href="#id40" name="x-problem-with-specification-of-a-m-in-readable-iterator">9.38x Problem with specification of a->m in Readable Iterator</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Howard Hinnant</td>
</tr>
<tr class="field"><th class="field-name">Status:</th><td class="field-body">New</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12585:</p>
<p>Readable Iterator Requirements says:</p>
<blockquote>
<table border class="table">
<colgroup>
<col width="13%" />
<col width="10%" />
<col width="77%" />
</colgroup>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">a->m</span></tt></td>
<td><tt class="literal"><span class="pre">U&</span></tt></td>
<td>pre: <tt class="literal"><span class="pre">(*a).m</span></tt> is well-defined. Equivalent to <tt class="literal"><span class="pre">(*a).m</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
<p>Do we mean to outlaw iterators with proxy references from meeting
the readable requirements?</p>
<p>Would it be better for the requirements to read <tt class="literal"><span class="pre">static_cast<T>(*a).m</span></tt>
instead of <tt class="literal"><span class="pre">(*a).m</span></tt> ?</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">NAD.</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body"><p class="first">We think you're misreading "pre:".
If <tt class="literal"><span class="pre">(*a).m</span></tt> is not well defined, then the iterator is not
required to provide <tt class="literal"><span class="pre">a->m</span></tt>. So a proxy iterator is not
required to provide <tt class="literal"><span class="pre">a->m</span></tt>.</p>
<p class="last">As an aside, it is possible for proxy iterators to
support <tt class="literal"><span class="pre">-></span></tt>, so changing the requirements to
read <tt class="literal"><span class="pre">static_cast<T>(*a).m</span></tt> is interesting.
However, such a change to Readable Iterator would
mean that it no longer corresponds to the
input iterator requirements. So old iterators would not
necessarily conform to new iterator requirements.</p>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="x-counting-iterator-traversal-argument-unspecified">
<h2><a class="toc-backref" href="#id41" name="x-counting-iterator-traversal-argument-unspecified">9.39x counting_iterator Traversal argument unspecified</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12635:</p>
<p>counting_iterator takes an argument for its Traversal type, with a
default value of use_default. It is derived from an instance of
iterator_adaptor, where the argument passed for the Traversal type
is described as "<tt class="literal"><span class="pre">/*</span> <span class="pre">see</span> <span class="pre">details</span> <span class="pre">for</span> <span class="pre">traversal</span> <span class="pre">category</span>
<span class="pre">*/</span></tt>". The details for counting_iterator describe constraints on
the Incrementable type imposed by various traversal
categories. There is no description of what the argument to
iterator_adaptor should be.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body">We no longer inherit from iterator_adaptor. So instead,
we specify the iterator_category in terms of the Traversal type
(which is now called CategoryOrTraversal). Also the
requirements and models section was reorganized to
match these changes and to make more sense.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="x-indirect-iterator-requirements-muddled">
<h2><a class="toc-backref" href="#id42" name="x-indirect-iterator-requirements-muddled">9.40x indirect_iterator requirements muddled</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12640:</p>
<blockquote>
<blockquote>
The value_type of the Iterator template parameter should itself
be dereferenceable. The return type of the <tt class="literal"><span class="pre">operator*</span></tt> for
the value_type must be the same type as the Reference template
parameter.</blockquote>
<p>I'd say this a bit differently, to emphasize what's required:
iterator_traits<Iterator>::value_type must be dereferenceable.
The Reference template parameter must be the same type as
<tt class="literal"><span class="pre">*iterator_traits<Iterator>::value_type()</span></tt>.</p>
<blockquote>
The Value template parameter will be the value_type for the
indirect_iterator, unless Value is const. If Value is const X, then
value_type will be non- const X.</blockquote>
<p>Also non-volatile, right? In other words, if Value isn't use_default, it
just gets passed as the Value argument for iterator_adaptor.</p>
<blockquote>
<p>The default for Value is:</p>
<pre class="literal-block">
iterator_traits< iterator_traits<Iterator>::value_type >::value_type
</pre>
<p>If the default is used for Value, then there must be a valid
specialization of iterator_traits for the value type of the
base iterator.</p>
</blockquote>
<p>The earlier requirement is that
<tt class="literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt> must be
dereferenceable. Now it's being treated as an iterator. Is this
just a pun, or is <tt class="literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt>
required to be some form of iterator? If it's the former we need
to find a different way to say it. If it's the latter we need to
say so.</p>
</blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change:</p>
<blockquote>
<p>The <tt class="literal"><span class="pre">value_type</span></tt> of the <tt class="literal"><span class="pre">Iterator</span></tt> template parameter
should itself be dereferenceable. The return type of the
<tt class="literal"><span class="pre">operator*</span></tt> for the <tt class="literal"><span class="pre">value_type</span></tt> must be the same type as
the <tt class="literal"><span class="pre">Reference</span></tt> template parameter. The <tt class="literal"><span class="pre">Value</span></tt> template
parameter will be the <tt class="literal"><span class="pre">value_type</span></tt> for the
<tt class="literal"><span class="pre">indirect_iterator</span></tt>, unless <tt class="literal"><span class="pre">Value</span></tt> is const. If <tt class="literal"><span class="pre">Value</span></tt>
is <tt class="literal"><span class="pre">const</span> <span class="pre">X</span></tt>, then <tt class="literal"><span class="pre">value_type</span></tt> will be <em>non-</em> <tt class="literal"><span class="pre">const</span> <span class="pre">X</span></tt>.
The default for <tt class="literal"><span class="pre">Value</span></tt> is:</p>
<pre class="literal-block">
iterator_traits< iterator_traits<Iterator>::value_type >::value_type
</pre>
<p>If the default is used for <tt class="literal"><span class="pre">Value</span></tt>, then there must be a
valid specialization of <tt class="literal"><span class="pre">iterator_traits</span></tt> for the value type
of the base iterator.</p>
<p>The <tt class="literal"><span class="pre">Reference</span></tt> parameter will be the <tt class="literal"><span class="pre">reference</span></tt> type of the
<tt class="literal"><span class="pre">indirect_iterator</span></tt>. The default is <tt class="literal"><span class="pre">Value&</span></tt>.</p>
<p>The <tt class="literal"><span class="pre">Access</span></tt> and <tt class="literal"><span class="pre">Traversal</span></tt> parameters are passed
unchanged to the corresponding parameters of the
<tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class, and the <tt class="literal"><span class="pre">Iterator</span></tt> parameter
is passed unchanged as the <tt class="literal"><span class="pre">Base</span></tt> parameter to the
<tt class="literal"><span class="pre">iterator_adaptor</span></tt> base class.</p>
</blockquote>
<p>to:</p>
<blockquote class="last">
<blockquote>
The expression <tt class="literal"><span class="pre">*v</span></tt>, where <tt class="literal"><span class="pre">v</span></tt> is an object of
<tt class="literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt>, shall be valid
expression and convertible to <tt class="literal"><span class="pre">reference</span></tt>. <tt class="literal"><span class="pre">Iterator</span></tt>
shall model the traversal concept indicated by
<tt class="literal"><span class="pre">iterator_category</span></tt>. <tt class="literal"><span class="pre">Value</span></tt>, <tt class="literal"><span class="pre">Reference</span></tt>, and
<tt class="literal"><span class="pre">Difference</span></tt> shall be chosen so that <tt class="literal"><span class="pre">value_type</span></tt>,
<tt class="literal"><span class="pre">reference</span></tt>, and <tt class="literal"><span class="pre">difference_type</span></tt> meet the requirements
indicated by <tt class="literal"><span class="pre">iterator_category</span></tt>.</blockquote>
<p>[Note: there are further requirements on the
<tt class="literal"><span class="pre">iterator_traits<Iterator>::value_type</span></tt> if the <tt class="literal"><span class="pre">Value</span></tt>
parameter is not <tt class="literal"><span class="pre">use_default</span></tt>, as implied by the algorithm
for deducing the default for the <tt class="literal"><span class="pre">value_type</span></tt> member.]</p>
</blockquote>
</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body">Not included above is the specification of the
<tt class="literal"><span class="pre">value_type</span></tt>, <tt class="literal"><span class="pre">reference</span></tt>, etc., members, which is handled by
the changes in 9.37x.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="x-problem-with-transform-iterator-requirements">
<h2><a class="toc-backref" href="#id43" name="x-problem-with-transform-iterator-requirements">9.41x Problem with transform_iterator requirements</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12641:</p>
<blockquote>
The reference type of transform_iterator is <tt class="literal"><span class="pre">result_of<</span>
<span class="pre">UnaryFunction(iterator_traits<Iterator>::reference)</span>
<span class="pre">>::type</span></tt>. The <tt class="literal"><span class="pre">value_type</span></tt> is
<tt class="literal"><span class="pre">remove_cv<remove_reference<reference></span> <span class="pre">>::type</span></tt>.</blockquote>
<p>These are the defaults, right? If the user supplies their own types
that's what gets passed to iterator_adaptor. And again, the
specification should be in terms of the specialization of
iterator_adaptor, and not in terms of the result:</p>
<p>Reference argument to iterator_adaptor:</p>
<pre class="literal-block">
if (Reference != use_default)
Reference
else
result_of<
UnaryFunction(iterator_traits<Iterator>::reference)
>::type
</pre>
<p>Value argument to iterator_adaptor:</p>
<pre class="literal-block">
if (Value != use_default)
Value
else if (Reference != use_default)
remove_reference<reference>::type
else
remove_reference<
result_of<
UnaryFunction(iterator_traits<Iterator>::reference)
>::type
>::type
</pre>
<p>There's probably a better way to specify that last alternative, but
I've been at this too long, and it's all turning into a maze of
twisty passages, all alike.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Replace:</p>
<blockquote>
The reference type of transform_iterator is <tt class="literal"><span class="pre">result_of<</span>
<span class="pre">UnaryFunction(iterator_traits<Iterator>::reference)</span>
<span class="pre">>::type</span></tt>. The <tt class="literal"><span class="pre">value_type</span></tt> is
<tt class="literal"><span class="pre">remove_cv<remove_reference<reference></span> <span class="pre">>::type</span></tt>.</blockquote>
<p>with:</p>
<blockquote class="last">
<p>If <tt class="literal"><span class="pre">Reference</span></tt> is <tt class="literal"><span class="pre">use_default</span></tt> then the <tt class="literal"><span class="pre">reference</span></tt>
member of <tt class="literal"><span class="pre">transform_iterator</span></tt> is <tt class="literal"><span class="pre">result_of<</span>
<span class="pre">UnaryFunction(iterator_traits<Iterator>::reference)</span>
<span class="pre">>::type</span></tt>. Otherwise, <tt class="literal"><span class="pre">reference</span></tt> is <tt class="literal"><span class="pre">Reference</span></tt>.</p>
<p>If <tt class="literal"><span class="pre">Value</span></tt> is <tt class="literal"><span class="pre">use_default</span></tt> then the <tt class="literal"><span class="pre">value_type</span></tt>
member is <tt class="literal"><span class="pre">remove_cv<remove_reference<reference></span> <span class="pre">>::type</span></tt>.
Otherwise, <tt class="literal"><span class="pre">value_type</span></tt> is <tt class="literal"><span class="pre">Value</span></tt>.</p>
</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="x-filter-iterator-details-unspecified">
<h2><a class="toc-backref" href="#id44" name="x-filter-iterator-details-unspecified">9.42x filter_iterator details unspecified</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Pete Becker</td>
</tr>
</tbody>
</table>
<p>c++std-lib-12642:</p>
<p>The paper says:</p>
<pre class="literal-block">
template<class Predicate, class Iterator>
class filter_iterator
: public iterator_adaptor<
filter_iterator<Predicate, Iterator>,
Iterator,
use_default,
/* see details */ >
</pre>
<p>That comment covers the Access, Traversal, Reference, and Difference
arguments. The only specification for any of these in the details is:</p>
<blockquote>
The access category of the filter_iterator will be the same as
the access category of Iterator.</blockquote>
<p>Needs more.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Add to the synopsis:</p>
<pre class="literal-block">
typedef iterator_traits<Iterator>::value_type value_type;
typedef iterator_traits<Iterator>::reference reference;
typedef iterator_traits<Iterator>::pointer pointer;
typedef iterator_traits<Iterator>::difference_type difference_type;
typedef /* see below */ iterator_category;
</pre>
<p>and add just after the synopsis:</p>
<blockquote class="last">
If <tt class="literal"><span class="pre">Iterator</span></tt> models Readable Lvalue Iterator and Forward
Traversal Iterator then <tt class="literal"><span class="pre">iterator_category</span></tt> is convertible
to <tt class="literal"><span class="pre">std::forward_iterator_tag</span></tt>. Otherwise
<tt class="literal"><span class="pre">iterator_category</span></tt> is convertible to
<tt class="literal"><span class="pre">std::input_iterator_tag</span></tt>.</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="x-transform-iterator-interoperability-too-restrictive">
<h2><a class="toc-backref" href="#id45" name="x-transform-iterator-interoperability-too-restrictive">9.43x transform_iterator interoperability too restrictive</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Jeremy Siek</td>
</tr>
</tbody>
</table>
<p>We do not need to require that the function objects have the same
type, just that they be convertible.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change:</p>
<pre class="literal-block">
template<class OtherIterator, class R2, class V2>
transform_iterator(
transform_iterator<UnaryFunction, OtherIterator, R2, V2> const& t
, typename enable_if_convertible<OtherIterator, Iterator>::type* = 0 // exposition
);
</pre>
<p>to:</p>
<pre class="last literal-block">
template<class F2, class I2, class R2, class V2>
transform_iterator(
transform_iterator<F2, I2, R2, V2> const& t
, typename enable_if_convertible<I2, Iterator>::type* = 0 // exposition only
, typename enable_if_convertible<F2, UnaryFunction>::type* = 0 // exposition only
);
</pre>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="y-indirect-iterator-and-smart-pointers">
<h2><a class="toc-backref" href="#id46" name="y-indirect-iterator-and-smart-pointers">9.44y <tt class="literal"><span class="pre">indirect_iterator</span></tt> and smart pointers</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">indirect_iterator</span></tt> should be able to iterate over containers of
smart pointers, but the mechanism that allows it was left out of
the specification, even though it's present in the Boost
specification</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Add <tt class="literal"><span class="pre">pointee</span></tt> and <tt class="literal"><span class="pre">indirect_reference</span></tt>
to deal with this capability.</p>
<p>In [lib.iterator.helper.synopsis], add:</p>
<pre class="literal-block">
template <class Dereferenceable>
struct pointee;
template <class Dereferenceable>
struct indirect_reference;
</pre>
<p class="last">After <tt class="literal"><span class="pre">indirect_iterator</span></tt>'s abstract, add:</p>
</td>
</tr>
</tbody>
</table>
<div class="topic">
<p class="topic-title">Class template <tt class="literal"><span class="pre">pointee</span></tt></p>
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<pre class="literal-block">
template <class Dereferenceable>
struct pointee
{
typedef /* see below */ type;
};
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body">For an object <tt class="literal"><span class="pre">x</span></tt> of type <tt class="literal"><span class="pre">Dereferenceable</span></tt>, <tt class="literal"><span class="pre">*x</span></tt>
is well-formed. If <tt class="literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
ambiguous nor shall it violate access control, and
<tt class="literal"><span class="pre">Dereferenceable::element_type</span></tt> shall be an accessible type.
Otherwise <tt class="literal"><span class="pre">iterator_traits<Dereferenceable>::value_type</span></tt> shall
be well formed. [Note: These requirements need not apply to
explicit or partial specializations of <tt class="literal"><span class="pre">pointee</span></tt>]</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
<tt class="literal"><span class="pre">x</span></tt> is an object of type <tt class="literal"><span class="pre">Dereferenceable</span></tt>:</p>
<pre class="literal-block">
if ( ++x is ill-formed )
{
return ``Dereferenceable::element_type``
}
else if (``*x`` is a mutable reference to
std::iterator_traits<Dereferenceable>::value_type)
{
return iterator_traits<Dereferenceable>::value_type
}
else
{
return iterator_traits<Dereferenceable>::value_type const
}
</pre>
</div>
<div class="topic">
<p class="topic-title">Class template <tt class="literal"><span class="pre">indirect_reference</span></tt></p>
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<pre class="literal-block">
template <class Dereferenceable>
struct indirect_reference
{
typedef /* see below */ type;
};
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Requires:</th><td class="field-body">For an object <tt class="literal"><span class="pre">x</span></tt> of type <tt class="literal"><span class="pre">Dereferenceable</span></tt>, <tt class="literal"><span class="pre">*x</span></tt>
is well-formed. If <tt class="literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
ambiguous nor shall it violate access control, and
<tt class="literal"><span class="pre">pointee<Dereferenceable>::type&</span></tt> shall be well-formed.
Otherwise <tt class="literal"><span class="pre">iterator_traits<Dereferenceable>::reference</span></tt> shall
be well formed. [Note: These requirements need not apply to
explicit or partial specializations of <tt class="literal"><span class="pre">indirect_reference</span></tt>]</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
<tt class="literal"><span class="pre">x</span></tt> is an object of type <tt class="literal"><span class="pre">Dereferenceable</span></tt>:</p>
<pre class="literal-block">
if ( ++x is ill-formed )
return ``pointee<Dereferenceable>::type&``
else
std::iterator_traits<Dereferenceable>::reference
</pre>
</div>
<p>See proposed resolution to Issue 9.37x for more changes related to
this issue.</p>
</div>
<div class="section" id="y-n1530-typos-and-editorial-changes-in-proposal-text-not-standardese">
<h2><a class="toc-backref" href="#id47" name="y-n1530-typos-and-editorial-changes-in-proposal-text-not-standardese">9.45y N1530: Typos and editorial changes in proposal text (not standardese)</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
</tbody>
</table>
<ol class="arabic">
<li><p class="first">"because specification helps to highlight that the <tt class="literal"><span class="pre">Reference</span></tt>
template parameter may not always be identical to the iterator's
<tt class="literal"><span class="pre">reference</span></tt> type, and will keep users making mistakes based on
that assumption."</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first last">add "from" before "making"</p>
</td>
</tr>
</tbody>
</table>
</li>
<li><p class="first">mention of obsolete projection_iterator</p>
</li>
</ol>
<blockquote>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed Resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">From n1530, in the <strong>Specialized Adaptors</strong> section, remove:</p>
<blockquote class="last">
<tt class="literal"><span class="pre">projection_iterator</span></tt>, which is similar to <tt class="literal"><span class="pre">transform_iterator</span></tt>
except that when dereferenced it returns a reference instead of
a value.</blockquote>
</td>
</tr>
<tr class="field"><th class="field-name">Rationale:</th><td class="field-body">This iterator was in the original boost library, but the new
iterator concepts allowed this iterator to be
folded into <tt class="literal"><span class="pre">transform_iterator</span></tt>.</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
<div class="section" id="y-n1530-base-return-by-value-is-costly">
<h2><a class="toc-backref" href="#id48" name="y-n1530-base-return-by-value-is-costly">9.46y N1530: <tt class="literal"><span class="pre">base()</span></tt> return-by-value is costly</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Dave Abrahams</td>
</tr>
</tbody>
</table>
<p>We've had some real-life reports that iterators that use
<tt class="literal"><span class="pre">iterator_adaptor</span></tt>'s <tt class="literal"><span class="pre">base()</span></tt> function can be inefficient
when the <tt class="literal"><span class="pre">Base</span></tt> iterator is expensive to copy. Iterators, of
all things, should be efficient.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">In [lib.iterator.adaptor]</p>
<p>Change:</p>
<pre class="literal-block">
Base base() const;
</pre>
<p>to:</p>
<pre class="literal-block">
Base const& base() const;
</pre>
<p class="last">twice (once in the synopsis and once in the <strong>public
operations</strong> section).</p>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="x-forgot-default-constructible-in-forward-traversal-iterator">
<h2><a class="toc-backref" href="#id49" name="x-forgot-default-constructible-in-forward-traversal-iterator">9.47x Forgot default constructible in Forward Traversal Iterator</a></h2>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Submitter:</th><td class="field-body">Jeremy Siek</td>
</tr>
</tbody>
</table>
<p>We want Forward Traversal Iterator plus Readable Lvalue Iterator to
match the old Foward Iterator requirements, so we need Forward
Traversal Iterator to include Default Constructible.</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Proposed resolution:</th></tr>
<tr><td> </td><td class="field-body"><p class="first">Change:</p>
<blockquote>
<p>A class or built-in type <tt class="literal"><span class="pre">X</span></tt> models the <em>Forward Traversal Iterator</em>
concept if the following expressions are valid and respect the stated
semantics.</p>
<table border class="table">
<colgroup>
<col width="44%" />
<col width="39%" />
<col width="17%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="3">Forward Traversal Iterator Requirements (in addition to Single Pass Iterator)</td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote class="last">
<p>A class or built-in type <tt class="literal"><span class="pre">X</span></tt> models the <em>Forward Traversal Iterator</em>
concept if, in addition to <tt class="literal"><span class="pre">X</span></tt> meeting the requirements of
Default Constructible and Single Pass Iterator, the following
expressions are valid and respect the
stated semantics.</p>
<table border class="table">
<colgroup>
<col width="38%" />
<col width="34%" />
<col width="27%" />
</colgroup>
<tbody valign="top">
<tr><td colspan="3">Forward Traversal Iterator Requirements (in addition to Default Constructible and Single Pass Iterator)</td>
</tr>
</tbody>
</table>
</blockquote>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="x-editorial-changes-non-normative-text">
<h2><a class="toc-backref" href="#id50" name="x-editorial-changes-non-normative-text">9.48x Editorial changes (non-normative text)</a></h2>
<dl>
<dt>Change:</dt>
<dd>Iterator facade uses the Curiously Recurring Template Pattern (CRTP)
[Cop95] so that the user can specify the behavior of
<tt class="literal"><span class="pre">iterator_facade</span></tt> in a derived class. Former designs used policy
objects to specify the behavior. <tt class="literal"><span class="pre">iterator_facade</span></tt> does not use policy
objects for several reasons:</dd>
<dt>to:</dt>
<dd>Iterator facade uses the Curiously Recurring Template
Pattern (CRTP) [Cop95] so that the user can specify the behavior
of <tt class="literal"><span class="pre">iterator_facade</span></tt> in a derived class. Former designs used
policy objects to specify the behavior, but that approach was
discarded for several reasons:</dd>
<dt>Change:</dt>
<dd>iterator's <tt class="literal"><span class="pre">operator++</span></tt> returns the iterator type itself means
that all iterators generated by <tt class="literal"><span class="pre">iterator_facade</span></tt> would be
instantiations of <tt class="literal"><span class="pre">iterator_facade</span></tt>. Cumbersome type generator</dd>
<dt>to:</dt>
<dd>iterator's <tt class="literal"><span class="pre">operator++</span></tt> returns the iterator type itself
would mean that all iterators built with the library would
have to be specializations of <tt class="literal"><span class="pre">iterator_facade<...></span></tt>, rather
than something more descriptive like
<tt class="literal"><span class="pre">indirect_iterator<T*></span></tt>. Cumbersome type generator</dd>
<dt>Change:</dt>
<dd>The return type for <tt class="literal"><span class="pre">operator-></span></tt> and <tt class="literal"><span class="pre">operator[]</span></tt> is not
explicitly specified. Instead it requires each <tt class="literal"><span class="pre">iterator_facade</span></tt>
instantiation to meet the requirements of its <tt class="literal"><span class="pre">iterator_category</span></tt>.</dd>
<dt>To:</dt>
<dd>The return types for <tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">operator-></span></tt> and
<tt class="literal"><span class="pre">operator[]</span></tt> are not explicitly specified. Instead, those types
are described in terms of a set of requirements, which must be
satisfied by the <tt class="literal"><span class="pre">iterator_facade</span></tt> implementation.</dd>
</dl>
</div>
<div class="section" id="x-clarification-of-iterator-facade-requirements-and-type-members">
<h2><a class="toc-backref" href="#id51" name="x-clarification-of-iterator-facade-requirements-and-type-members">9.49x Clarification of iterator_facade requirements and type members</a></h2>
<p>A general cleanup and simplification of the requirements and
description of type members for <tt class="literal"><span class="pre">iterator_facade</span></tt>.</p>
<p>The user is only allowed to add <tt class="literal"><span class="pre">const</span></tt> as a qualifier.</p>
<dl>
<dt>Change:</dt>
<dd><tt class="literal"><span class="pre">typedef</span> <span class="pre">remove_cv<Value>::type</span> <span class="pre">value_type;</span></tt></dd>
<dt>to:</dt>
<dd><tt class="literal"><span class="pre">typedef</span> <span class="pre">remove_const<Value>::type</span> <span class="pre">value_type;</span></tt></dd>
</dl>
<p>We use to have an unspecified type for <tt class="literal"><span class="pre">pointer</span></tt>, to match the
return type of <tt class="literal"><span class="pre">operator-></span></tt>, but there's no real reason to make them
match, so we just use the simpler <tt class="literal"><span class="pre">Value*</span></tt> for <tt class="literal"><span class="pre">pointer</span></tt>.</p>
<p>Change:</p>
<blockquote>
<tt class="literal"><span class="pre">typedef</span> <span class="pre">/*</span> <span class="pre">see</span> <span class="pre">description</span> <span class="pre">of</span> <span class="pre">operator-></span> <span class="pre">*/</span> <span class="pre">pointer;</span></tt></blockquote>
<dl>
<dt>To:</dt>
<dd><tt class="literal"><span class="pre">typedef</span> <span class="pre">Value*</span> <span class="pre">pointer;</span></tt></dd>
<dt>Remove:</dt>
<dd>Some of the constraints on template parameters to
<tt class="literal"><span class="pre">iterator_facade</span></tt> are expressed in terms of resulting nested
types and should be viewed in the context of their impact on
<tt class="literal"><span class="pre">iterator_traits<Derived></span></tt>.</dd>
<dt>Change:</dt>
<dd>The <tt class="literal"><span class="pre">Derived</span></tt> template parameter must be a class derived from
<tt class="literal"><span class="pre">iterator_facade</span></tt>.</dd>
<dt>and:</dt>
<dd>The following table describes the other requirements on the
<tt class="literal"><span class="pre">Derived</span></tt> parameter. Depending on the resulting iterator's
<tt class="literal"><span class="pre">iterator_category</span></tt>, a subset of the expressions listed in the table
are required to be valid. The operations in the first column must be
accessible to member functions of class <tt class="literal"><span class="pre">iterator_core_access</span></tt>.</dd>
<dt>to:</dt>
<dd>The following table describes the typical valid expressions on
<tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">Derived</span></tt> parameter, depending on the
iterator concept(s) it will model. The operations in the first
column must be made accessible to member functions of class
<tt class="literal"><span class="pre">iterator_core_access</span></tt>. In addition,
<tt class="literal"><span class="pre">static_cast<Derived*>(iterator_facade*)</span></tt> shall be well-formed.</dd>
<dt>Remove:</dt>
<dd><p class="first">The nested <tt class="literal"><span class="pre">::value_type</span></tt> type will be the same as
<tt class="literal"><span class="pre">remove_cv<Value>::type</span></tt>, so the <tt class="literal"><span class="pre">Value</span></tt> parameter must be
an (optionally <tt class="literal"><span class="pre">const</span></tt>-qualified) non-reference type.</p>
<p class="last">The nested <tt class="literal"><span class="pre">::reference</span></tt> will be the same as the <tt class="literal"><span class="pre">Reference</span></tt>
parameter; it must be a suitable reference type for the resulting
iterator. The default for the <tt class="literal"><span class="pre">Reference</span></tt> parameter is
<tt class="literal"><span class="pre">Value&</span></tt>.</p>
</dd>
</dl>
<p>Change:</p>
<blockquote>
<p>In the table below, <tt class="literal"><span class="pre">X</span></tt> is the derived iterator type, <tt class="literal"><span class="pre">a</span></tt> is an
object of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">b</span></tt> and <tt class="literal"><span class="pre">c</span></tt> are objects of type <tt class="literal"><span class="pre">const</span> <span class="pre">X</span></tt>,
<tt class="literal"><span class="pre">n</span></tt> is an object of <tt class="literal"><span class="pre">X::difference_type</span></tt>, <tt class="literal"><span class="pre">y</span></tt> is a constant
object of a single pass iterator type interoperable with X, and <tt class="literal"><span class="pre">z</span></tt>
is a constant object of a random access traversal iterator type
interoperable with <tt class="literal"><span class="pre">X</span></tt>.</p>
<table border class="table">
<colgroup>
<col width="19%" />
<col width="18%" />
<col width="36%" />
<col width="26%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Note</th>
<th>Required to implement
Iterator Concept(s)</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.dereference()</span></tt></td>
<td><tt class="literal"><span class="pre">X::reference</span></tt></td>
<td> </td>
<td>Readable Iterator, Writable
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.equal(b)</span></tt></td>
<td>convertible to bool</td>
<td>true iff <tt class="literal"><span class="pre">b</span></tt> and <tt class="literal"><span class="pre">c</span></tt> are
equivalent.</td>
<td>Single Pass Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.equal(y)</span></tt></td>
<td>convertible to bool</td>
<td>true iff <tt class="literal"><span class="pre">c</span></tt> and <tt class="literal"><span class="pre">y</span></tt> refer to the
same position. Implements <tt class="literal"><span class="pre">c</span> <span class="pre">==</span> <span class="pre">y</span></tt>
and <tt class="literal"><span class="pre">c</span> <span class="pre">!=</span> <span class="pre">y</span></tt>.</td>
<td>Single Pass Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.advance(n)</span></tt></td>
<td>unused</td>
<td> </td>
<td>Random Access Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.increment()</span></tt></td>
<td>unused</td>
<td> </td>
<td>Incrementable Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.decrement()</span></tt></td>
<td>unused</td>
<td> </td>
<td>Bidirectional Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.distance_to(b)</span></tt></td>
<td>convertible to
X::difference_type</td>
<td>equivalent to <tt class="literal"><span class="pre">distance(c,</span> <span class="pre">b)</span></tt></td>
<td>Random Access Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.distance_to(z)</span></tt></td>
<td>convertible to
X::difference_type</td>
<td>equivalent to <tt class="literal"><span class="pre">distance(c,</span> <span class="pre">z)</span></tt>.
Implements <tt class="literal"><span class="pre">c</span> <span class="pre">-</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span> <span class="pre"><</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span>
<span class="pre"><=</span> <span class="pre">z</span></tt>, <tt class="literal"><span class="pre">c</span> <span class="pre">></span> <span class="pre">z</span></tt>, and <tt class="literal"><span class="pre">c</span> <span class="pre">>=</span> <span class="pre">c</span></tt>.</td>
<td>Random Access Traversal
Iterator</td>
</tr>
</tbody>
</table>
</blockquote>
<p>to:</p>
<blockquote>
<p>In the table below, <tt class="literal"><span class="pre">F</span></tt> is <tt class="literal"><span class="pre">iterator_facade<X,V,C,R,D></span></tt>, <tt class="literal"><span class="pre">a</span></tt> is an
object of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">b</span></tt> and <tt class="literal"><span class="pre">c</span></tt> are objects of type <tt class="literal"><span class="pre">const</span> <span class="pre">X</span></tt>,
<tt class="literal"><span class="pre">n</span></tt> is an object of <tt class="literal"><span class="pre">F::difference_type</span></tt>, <tt class="literal"><span class="pre">y</span></tt> is a constant
object of a single pass iterator type interoperable with <tt class="literal"><span class="pre">X</span></tt>, and <tt class="literal"><span class="pre">z</span></tt>
is a constant object of a random access traversal iterator type
interoperable with <tt class="literal"><span class="pre">X</span></tt>.</p>
<p><strong>iterator_facade Core Operations</strong></p>
<table border class="table">
<colgroup>
<col width="21%" />
<col width="23%" />
<col width="27%" />
<col width="29%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Note</th>
<th>Used to implement Iterator
Concept(s)</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.dereference()</span></tt></td>
<td><tt class="literal"><span class="pre">F::reference</span></tt></td>
<td> </td>
<td>Readable Iterator, Writable
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.equal(y)</span></tt></td>
<td>convertible to bool</td>
<td>true iff <tt class="literal"><span class="pre">c</span></tt> and <tt class="literal"><span class="pre">y</span></tt>
refer to the same
position.</td>
<td>Single Pass Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.increment()</span></tt></td>
<td>unused</td>
<td> </td>
<td>Incrementable Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.decrement()</span></tt></td>
<td>unused</td>
<td> </td>
<td>Bidirectional Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.advance(n)</span></tt></td>
<td>unused</td>
<td> </td>
<td>Random Access Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.distance_to(z)</span></tt></td>
<td>convertible to
<tt class="literal"><span class="pre">F::difference_type</span></tt></td>
<td>equivalent to
<tt class="literal"><span class="pre">distance(c,</span> <span class="pre">X(z))</span></tt>.</td>
<td>Random Access Traversal
Iterator</td>
</tr>
</tbody>
</table>
</blockquote>
</div>
</div>
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