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<h1>Member Function Adapters</h1>
<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
includes improved versions of the full range of member function adapters
from the the C++ Standard Library (§20.3.8):</p>
<ul>
<li><tt>mem_fun_t</tt></li>
<li><tt>mem_fun1_t</tt></li>
<li><tt>const_mem_fun_t</tt></li>
<li><tt>const_mem_fun1_t</tt></li>
<li><tt>mem_fun_ref_t</tt></li>
<li><tt>mem_fun1_ref_t</tt></li>
<li><tt>const_mem_fun_ref_t</tt></li>
<li><tt>const_mem_fun1_ref_t</tt></li>
</ul>
<p>as well as the corresponding overloaded helper functions</p>
<ul>
<li><tt>mem_fun</tt></li>
<li><tt>mem_fun_ref</tt></li>
</ul>
<p>The following changes have been made to the adapters as specified in the
Standard:</p>
<ul>
<li>The <tt>first_argument_type</tt> typedef has been corrected for the
<tt>const_</tt> family of member function adapters (see <a href=
"#firstarg">below</a>).</li>
<li>The argument passed to <tt>mem_fun1_t</tt> and its variants is passed
using the <tt>call_traits::param_type</tt> for the member function's
argument type.</li>
</ul>
<h3 id="firstarg">first_argument_type</h3>
<p>The standard specifies <tt>const_mem_fun1_t</tt>, for example, like
this:</p>
<blockquote>
<pre>
template <class S, class T, class A> class const_mem_fun1_t
: public binary_function<<strong>T*</strong>, A, S> {
public:
explicit const_mem_fun1_t(S (T::*p)(A) const);
S operator()(<strong>const T*</strong> p, A x) const;
};
</pre>
</blockquote>
<p>Note that the first argument to <tt>binary_function</tt> is <tt>T*</tt>
despite the fact that the first argument to <tt>operator()</tt> is actually
of type <tt><em>const</em> T*</tt>.</p>
<p>Does this matter? Well, consider what happens when we write</p>
<blockquote>
<pre>
struct Foo { void bar(int) const; };
const Foo *cp = new Foo;
std::bind1st(std::mem_fun(&Foo::bar), cp);
</pre>
</blockquote>
<p>We have created a <tt>const_mem_fun1_t</tt> object which will
effectively contain the following</p>
<blockquote>
<pre>
typedef Foo* first_argument_type;
</pre>
</blockquote>
<p>The <tt>bind1st</tt> will then create a <tt>binder1st</tt> object that
will use this <tt>typedef</tt> as the type of a member which will be
initialised with <tt>cp</tt>. In other words, we will need to initialise a
<tt>Foo*</tt> member with a <tt>const Foo*</tt> pointer! Clearly this
is not possible, so to implement this your Standard Library vendor will
have had to cast away the constness of <tt>cp</tt>, probably within the
body of <tt>bind1st</tt>.</p>
<p>This hack will not suffice with the improved <a href=
"binders.html">binders</a> in this library, so we have had to provide
corrected versions of the member function adapters as well.</p>
<h3 id="args">Argument Types</h3>
<p>The standard defines <tt>mem_fun1_t</tt>, for example, like this
(§20.3.8 ¶2):</p>
<blockquote>
<pre>
template <class S, class T, class A> class mem_fun1_t
: public binary_function<T*, A, S> {
public:
explicit mem_fun1_t(S (T::*p)(<strong>A</strong>));
S operator()(T* p, <strong>A</strong> x) const;
};
</pre>
</blockquote>
<p>Note that the second argument to <tt>operator()</tt> is exactly the same
type as the argument to the member function. If this is a value type, the
argument will be passed by value and copied twice.</p>
<p>However, if we were to try and eliminate this inefficiency by instead
declaring the argument as <tt>const A&</tt>, then if A were a
reference type, we would have a reference to a reference, which is
currently illegal (but see <a href=
"http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#106">C++ core
language issue number 106)</a></p>
<p>So the way in which we want to declare the second argument for
<tt>operator()</tt> depends on whether or not the member function's
argument is a reference. If it is a reference, we want to declare it simply
as <tt>A</tt>; if it is a value we want to declare it as
<tt>const A&</tt>.</p>
<p>The Boost <a href="../utility/call_traits.htm">call_traits</a> class
template contains a <tt>param_type</tt> typedef, which uses partial
specialisation to make precisely this decision. By declaring the
<tt>operator()</tt> as</p>
<blockquote>
<pre>
S operator()(T* p, typename call_traits<A>::param_type x) const
</pre>
</blockquote>
<p>we achieve the desired result - we improve efficiency without generating
references to references.</p>
<h3>Limitations</h3>
<p>The call traits template used to realise some improvements relies on
partial specialisation, so these improvements are only available on
compilers that support that feature. With other compilers, the argument
passed to the member function (in the <tt>mem_fun1_t</tt> family) will
always be passed by reference, thus generating the possibility of
references to references.</p>
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<p>Revised
<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02 December, 2006<!--webbot bot="Timestamp" endspan i-checksum="38510" --></p>
<p><i>Copyright © 2000 Cadenza New Zealand Ltd.</i></p>
<p><i>Distributed under the Boost Software License, Version 1.0. (See
accompanying file <a href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</a> or
copy at <a href=
"http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</i></p>
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