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- <h1>Function Pointer Adapters</h1>
- <p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
- provides enhanced versions of both the function pointer adapters from the
- C++ Standard Library (§20.3.7):</p>
- <ul>
- <li><tt>pointer_to_unary_function</tt></li>
- <li><tt>pointer_to_binary_function</tt></li>
- </ul>
- <p>As well as the corresponding helper function template:</p>
- <ul>
- <li><tt>ptr_fun</tt></li>
- </ul>
- <p>However, you should not need to use the adapters in conjunction with the
- adapters in this library due to our use of <a href=
- "function_traits.html">function object traits</a>. You will however need to
- use them if your implementation fails to work properly with our traits
- classes (due to lack if partial specialisation), or if you wish to use a
- function object adapter from a third party.</p>
- <h3>Usage</h3>
- <p>If you need to use these adapters, usage is identical to the standard
- function pointer adapters. For example,</p>
- <blockquote>
- <pre>
- bool bad(std::string foo) { ... }
- ...
- std::vector<std::string> c;
- ...
- std::vector<std::string>::iterator it
- = std::find_if(c.begin(), c.end(), std::not1(boost::ptr_fun(bad)));
- </pre>
- </blockquote>
- <p>Note however that this library contains enhanced <a href=
- "negators.html">negators</a> that support function object traits, so the
- line above could equally be written</p>
- <blockquote>
- <pre>
- std::vector<std::string>::iterator it
- = std::find_if(c.begin(), c.end(), boost::not1(bad));
- </pre>
- </blockquote>
- <h3>Argument Types</h3>
- <p>The standard defines <tt>pointer_to_unary_function</tt> like this
- (§20.3.8 ¶2):</p>
- <blockquote>
- <pre>
- template <class Arg, class Result>
- class pointer_to_unary_function : public unary_function<Arg, Result> {
- public:
- explicit pointer_to_unary_function(Result (* f)(<strong>Arg</strong>));
- Result operator()(<strong>Arg</strong> x) const;
- };
- </pre>
- </blockquote>
- <p>Note that the argument to <tt>operator()</tt> is exactly the same type
- as the argument to the wrapped function. If this is a value type, the
- argument will be passed by value and copied twice.
- <tt>pointer_to_binary_function</tt> has a similar problem.</p>
- <p>However, if we were to try and eliminate this inefficiency by instead
- declaring the argument as <tt>const Arg&</tt>, then if Arg were a
- reference type, we would have a reference to a reference, which is
- currently illegal (but see <a href=
- "http://www.open-std.org/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 argument for
- <tt>operator()</tt> depends on whether or not the wrapped function's
- argument is a reference. If it is a reference, we want to declare it simply
- as <tt>Arg</tt>; if it is a value we want to declare it as
- <tt>const Arg&</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>
- Result operator()(typename call_traits<Arg>::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 this improvement relies on
- partial specialisation, so this improvement is only available on compilers
- that support that feature. With other compilers, the argument passed to the
- function 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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