159 lines
5.5 KiB
HTML
159 lines
5.5 KiB
HTML
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<title>Boost Function Object Adapter Library</title>
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<h1>Function Pointer Adapters</h1>
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<p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
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provides enhanced versions of both the function pointer adapters from the
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C++ Standard Library (§20.3.7):</p>
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<ul>
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<li><tt>pointer_to_unary_function</tt></li>
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<li><tt>pointer_to_binary_function</tt></li>
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</ul>
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<p>As well as the corresponding helper function template:</p>
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<ul>
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<li><tt>ptr_fun</tt></li>
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</ul>
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<p>However, you should not need to use the adapters in conjunction with the
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adapters in this library due to our use of <a href=
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"function_traits.html">function object traits</a>. You will however need to
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use them if your implementation fails to work properly with our traits
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classes (due to lack if partial specialisation), or if you wish to use a
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function object adapter from a third party.</p>
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<h3>Usage</h3>
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<p>If you need to use these adapters, usage is identical to the standard
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function pointer adapters. For example,</p>
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<blockquote>
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<pre>
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bool bad(std::string foo) { ... }
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...
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std::vector<std::string> c;
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...
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std::vector<std::string>::iterator it
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= std::find_if(c.begin(), c.end(), std::not1(boost::ptr_fun(bad)));
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</pre>
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</blockquote>
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<p>Note however that this library contains enhanced <a href=
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"negators.html">negators</a> that support function object traits, so the
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line above could equally be written</p>
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<blockquote>
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<pre>
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std::vector<std::string>::iterator it
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= std::find_if(c.begin(), c.end(), boost::not1(bad));
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</pre>
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</blockquote>
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<h3>Argument Types</h3>
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<p>The standard defines <tt>pointer_to_unary_function</tt> 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 Arg, class Result>
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class pointer_to_unary_function : public unary_function<Arg, Result> {
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public:
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explicit pointer_to_unary_function(Result (* f)(<strong>Arg</strong>));
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Result operator()(<strong>Arg</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 argument to <tt>operator()</tt> is exactly the same type
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as the argument to the wrapped function. If this is a value type, the
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argument will be passed by value and copied twice.
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<tt>pointer_to_binary_function</tt> has a similar problem.</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 Arg&</tt>, then if Arg 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 argument for
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<tt>operator()</tt> depends on whether or not the wrapped 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>Arg</tt>; if it is a value we want to declare it as
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<tt>const Arg&</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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Result operator()(typename call_traits<Arg>::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 this improvement relies on
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partial specialisation, so this improvement is only available on compilers
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that support that feature. With other compilers, the argument passed to the
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function will always be passed by reference, thus generating the
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possibility of references to references.</p>
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<hr>
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<p><a href="http://validator.w3.org/check?uri=referer"><img border="0" src=
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height="31" width="88"></a></p>
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<p>Revised
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<!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->02
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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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