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  <h1>Binders</h1>

  <p>The header <a href="../../boost/functional.hpp">functional.hpp</a>
  provides enhanced versions of both the binder function object adapters from
  the C++ Standard Library (&sect;20.3.6):</p>

  <ul>
    <li><tt>binder1st</tt></li>

    <li><tt>binder2nd</tt></li>
  </ul>

  <p>As well as the corresponding helper functions</p>

  <ul>
    <li><tt>bind1st</tt></li>

    <li><tt>bind2nd</tt></li>
  </ul>

  <p>The key benefit of these adapters over those in the Standard Library is
  they avoid the problem of <a href="#refref">references to
  references.</a></p>

  <h3>Usage</h3>

  <p>Usage is identical to the standard binders. For example,</p>

  <blockquote>
    <pre>
class Foo {
public:
  void bar(std::ostream &amp;);
  // ...
};
// ...
std::vector&lt;Foo&gt; c;
// ...
std::for_each(c.begin(), c.end(), 
              boost::bind2nd(boost::mem_fun_ref(&amp;Foo::bar), std::cout));
</pre>
  </blockquote>

  <h3 id="refref">References to References</h3>

  <p>Consider the usage example above</p>

  <blockquote>
    <pre>
class Foo {
public:
  void bar(<strong>std::ostream &amp;</strong>);
  // ...
};
// ...
std::for_each(c.begin(), c.end(), 
              boost::bind2nd(boost::mem_fun_ref(&amp;Foo::bar), std::cout));
</pre>
  </blockquote>

  <p>If this had been written using <tt>std::bind2nd</tt> and
  <tt>std::mem_fun_ref</tt>, it would be unlikely to compile.</p>

  <p>The problem arises because <tt>bar</tt> takes a reference argument. The
  Standard defines <tt>std::mem_fun_ref</tt> such that it creates a function
  object whose <tt>second_argument_type</tt> will be
  <tt>std::ostream&amp;</tt>.</p>

  <p>The call to <tt>bind2nd</tt> creates a <tt>binder2nd</tt> which the
  Standard defines as follows:</p>

  <blockquote>
    <pre>
template &lt;class Operation&gt;
class binder2nd
    : public unary_function&lt;typename Operation::first_argument_type,
                            typename Operation::result_type&gt; {
...
public:
  binder2nd(const Operation&amp; x,
            <strong>const typename Operation::second_argument_type&amp; y</strong>);
  ...
</pre>
  </blockquote>

  <p>Since our operation's <tt>second_argument_type</tt> is
  <tt>std::ostream&amp;</tt>, the type of <tt>y</tt> in the constructor would
  be <tt>std::ostream&amp;&amp;</tt>. Since you cannot have a reference to a
  reference, at this point we should get a compilation error because
  references to references are illegal in C++ (but see <a href=
  "http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#106">C++
  Standard core language active issues list</a>).</p>

  <p>The binders in this library avoid this problem by using the Boost
  <tt><a href="../utility/call_traits.htm">call_traits</a></tt>
  templates.</p>

  <p>Our constructor is declared</p>

  <blockquote>
    <pre>
binder2nd(const Operation&amp; x,
          <strong>typename call_traits&lt;
             typename binary_traits&lt;Operation&gt;::second_argument_type
          &gt;::param_type y</strong>)
</pre>
  </blockquote>

  <p>As a result, <tt>y</tt> has a type of <tt>std::ostream&amp;</tt>, and
  our example compiles.</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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