boost_1_45_0/libs/functional/ptr_fun.html

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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 (&sect;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&lt;std::string&gt; c;
...
std::vector&lt;std::string&gt;::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&lt;std::string&gt;::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
  (&sect;20.3.8&nbsp;&para;2):</p>

  <blockquote>
    <pre>
template &lt;class Arg, class Result&gt;
class pointer_to_unary_function : public unary_function&lt;Arg, Result&gt; {
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&nbsp;Arg&amp;</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://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 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&nbsp;Arg&amp;</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&lt;Arg&gt;::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 &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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