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[SVN r36246]
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Andreas Huber
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<h1>Member Function Adapters</h1>
<td><a href="../libraries.htm"><font face="Arial" color=
"#FFFFFF"><big>Libraries</big></font></a></td>
<p>The header <nobr><a
href="../../boost/functional.hpp">functional.hpp</a></nobr> includes
improved versions of the full range of member function adapters from
the the C++ Standard Library <nobr>(&sect 20.3.8):</nobr></p>
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<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>
<td><a href="../../more/faq.htm"><font face="Arial" color=
"#FFFFFF"><big>FAQ</big></font></a></td>
<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>
<td><a href="../../more/index.htm"><font face="Arial" color=
"#FFFFFF"><big>More</big></font></a></td>
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<p>The following changes have been made to the adapters as specified
in the Standard:</p>
<h1>Member Function Adapters</h1>
<ul>
<li>The <tt><nobr>first_argument_type</nobr></tt> typedef has been
corrected for the <nobr><tt>const_</tt></nobr> family of member
function adapters (see <a href="#firstarg">below</a>).</li>
<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 (&sect;20.3.8):</p>
<li>The argument passed to <tt><nobr>mem_fun1_t</nobr></tt> and its
variants is passed using the
<tt><nobr>call_traits::param_type</nobr></tt> for the member
function's argument type.
</ul>
<ul>
<li><tt>mem_fun_t</tt></li>
<h3 id="firstarg">first_argument_type</h3>
<li><tt>mem_fun1_t</tt></li>
<p>The standard specifies <tt><nobr>const_mem_fun1_t</nobr></tt>, for example, like this:
<li><tt>const_mem_fun_t</tt></li>
<blockquote><pre>
<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 &lt;class S, class T, class A&gt; class const_mem_fun1_t
: public binary_function&lt;<strong>T*</strong>, A, S&gt; {
public:
explicit const_mem_fun1_t(S (T::*p)(A) const);
S operator()(<strong>const T*</strong> p, A x) const;
};
</pre></blockquote>
</pre>
</blockquote>
<p>Note that the first argument to
<tt><nobr>binary_function</nobr></tt> is <tt><nobr>T*</nobr></tt>
despite the fact that the first argument to <tt><nobr>operator()</nobr></tt> is
actually of type <tt><nobr><em>const</em> T*</nobr></tt>.
<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>&nbsp;T*</tt>.</p>
<p>Does this matter? Well, consider what happens when we write
<p>Does this matter? Well, consider what happens when we write</p>
<blockquote><pre>
<blockquote>
<pre>
struct Foo { void bar(int) const; };
const Foo *cp = new Foo;
std::bind1st(std::mem_fun(&Foo::bar), cp);
</pre></blockquote>
std::bind1st(std::mem_fun(&amp;Foo::bar), cp);
</pre>
</blockquote>
<p>We have created a <tt><nobr>const_mem_fun1_t</nobr></tt> object
which will effectively contain the following
<p>We have created a <tt>const_mem_fun1_t</tt> object which will
effectively contain the following</p>
<blockquote><pre>
<blockquote>
<pre>
typedef Foo* first_argument_type;
</pre></blockquote>
</pre>
</blockquote>
<p>The <tt><nobr>bind1st</nobr></tt> will then create a
<tt><nobr>binder1st</nobr></tt> object that will use this
<tt><nobr>typedef</nobr></tt> as the type of a member which will be
initialised with <tt><nobr>cp</nobr></tt>. In other words, we will
need to initialise a <tt><nobr>Foo*</nobr></tt> member with a
<tt><nobr>const Foo*</nobr></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><nobr>cp</nobr></tt>, probably
within the body of <tt><nobr>bind1st</nobr></tt>.
<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&nbsp;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>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>
<h3 id="args">Argument Types</h3>
<p>The standard defines <tt>mem_fun1_t</tt>, for example, like this
(&sect;20.3.8&nbsp;&para;2):</p>
<p>The standard defines <nobr><tt>mem_fun1_t</tt></nobr>, for example, like this
<nobr>(&sect;20.3.8 &para;2):</nobr>
<blockquote><pre>
<blockquote>
<pre>
template &lt;class S, class T, class A&gt; class mem_fun1_t
: public binary_function&lt;T*, A, S&gt; {
public:
explicit mem_fun1_t(S (T::*p)(<strong>A</strong>));
S operator()(T* p, <strong>A</strong> x) const;
};
</pre></blockquote>
</pre>
</blockquote>
<p>Note that the second argument to <nobr><tt>operator()</tt></nobr> 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>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 <nobr><tt>const A&</tt></nobr>, 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>However, if we were to try and eliminate this inefficiency by instead
declaring the argument as <tt>const&nbsp;A&amp;</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
<nobr><tt>operator()</tt></nobr> 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 <nobr><tt>A</tt></nobr>; if it is a value we want
to declare it as <nobr><tt>const A&</tt></nobr>.
<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&nbsp;A&amp;</tt>.</p>
<p>The Boost <nobr><a
href="../utility/call_traits.htm">call_traits</a></nobr> class
template contains a <tt><nobr>param_type</nobr></tt> typedef, which
uses partial specialisation to make precisely this decision. By
declaring the <nobr><tt>operator()</tt></nobr> as
<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>
<blockquote>
<pre>
S operator()(T* p, typename call_traits&lt;A&gt;::param_type x) const
</pre></blockquote>
</pre>
</blockquote>
<p>we achieve the desired result - we improve efficiency without
generating references to references.</p>
<p>we achieve the desired result - we improve efficiency without generating
references to references.</p>
<h3>Limitations</h3>
<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
<nobr><tt>mem_fun1_t</tt></nobr> family) will always be passed by
reference, thus generating the possibility of references to references.
<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>Copyright &copy; 2000 Cadenza New Zealand Ltd. Permission to copy,
use, modify, sell and distribute this document is granted provided
this copyright notice appears in all copies. This document is provided
"as is" without express or implied warranty, and with no claim as to
its suitability for any purpose.</p>
<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>Revised 28 June 2000</p>
<p><i>Copyright &copy; 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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