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