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<Title>MultiPassInputIterator</Title>
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<H2>
<A NAME="concept:MultiPassInputIterator"></A>
Multi-Pass Input Iterator
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This concept is a refinement of <a
href="http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>,
adding the requirements that the iterator can be used to make multiple
passes through a range, and that if <TT>it1 == it2</TT> and
<TT>it1</TT> is dereferenceable then <TT>++it1 == ++it2</TT>. The
Multi-Pass Input Iterator is very similar to the <a
href="http://www.sgi.com/tech/stl/ForwardIterator.hmtl">Forward Iterator</a>. The
only difference is that a <a
href="http://www.sgi.com/tech/stl/ForwardIterator.hmtl">Forward Iterator</a>
requires the <TT>reference</TT> type to be <TT>value_type&</TT>, whereas
MultiPassInputIterator is like <a
href="http://www.sgi.com/tech/stl/InputIterator.html">Input Iterator</a>
in that the <TT>reference</TT> type merely has to be convertible to
<TT>value_type</TT>.
<h3>Design Notes</h3>
comments by Valentin Bonnard:
<p> I think that introducing Multi-Pass Input Iterator isn't the right
solution. Do you also want to define Multi-Pass Bidirectionnal Iterator
and Multi-Pass Random Access Iterator ? I don't, definitly. It only
confuses the issue. The problem lies into the existing hierarchy of
iterators, which mixes movabillity, modifiabillity and lvalue-ness,
and these are clearly independant.
<p> The terms Forward, Bidirectionnal and Random Access are about
movabillity and shouldn't be used to mean anything else. In a
completly orthogonal way, iterators can be immutable, mutable, or
neither. Lvalueness of iterators is also orthogonal with
immutabillity. With these clean concepts, your Multi-Pass Input Iterator
is just called a Forward Iterator.
<p>
Other translations are:<br>
std::Forward Iterator -> ForwardIterator & Lvalue Iterator<br>
std::Bidirectionnal Iterator -> Bidirectionnal Iterator & Lvalue Iterator<br>
std::Random Access Iterator -> Random Access Iterator & Lvalue Iterator<br>
<p>
Note that in practice the only operation not allowed on my
Forward Iterator which is allowed on std::Forward Iterator is
<tt>&*it</tt>. I think that <tt>&*</tt> is rarely needed in generic code.
<p>
reply by Jeremy Siek:
<p>
The above analysis by Valentin is right on. Of course, there is
the problem with backward compatibility. The current STL implementations
are based on the old definition of Forward Iterator. The right course
of action is to get Forward Iterator, etc. changed in the C++ standard.
Once that is done we can drop Multi-Pass Input Iterator.
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<TD nowrap>Copyright © 2000</TD><TD>
<a HREF="../../people/jeremy_siek.htm">Jeremy Siek</a>, Univ.of Notre Dame (<A HREF="mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</A>)
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