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  12   Extensions
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  15     The <code class="filename">&lt;functional&gt;</code> header
  16     contains many additional functors
  17     and helper functions, extending section 20.3.  They are
  18     implemented in the file stl_function.h:
  19   </p><div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"><p><code class="code">identity_element</code> for addition and multiplication.
  20   </p></li><li class="listitem"><p>The functor <code class="code">identity</code>, whose <code class="code">operator()</code>
  21       returns the argument unchanged.
  22   </p></li><li class="listitem"><p>Composition functors <code class="code">unary_function</code> and
  23       <code class="code">binary_function</code>, and their helpers <code class="code">compose1</code>
  24       and <code class="code">compose2</code>.
  25     </p></li><li class="listitem"><p><code class="code">select1st</code> and <code class="code">select2nd</code>, to strip pairs.
  26   </p></li><li class="listitem"><p><code class="code">project1st</code> and <code class="code">project2nd</code>. </p></li><li class="listitem"><p>A set of functors/functions which always return the same result.  They
  27       are <code class="code">constant_void_fun</code>, <code class="code">constant_binary_fun</code>,
  28       <code class="code">constant_unary_fun</code>, <code class="code">constant0</code>,
  29       <code class="code">constant1</code>, and <code class="code">constant2</code>. </p></li><li class="listitem"><p>The class <code class="code">subtractive_rng</code>. </p></li><li class="listitem"><p>mem_fun adaptor helpers <code class="code">mem_fun1</code> and
  30       <code class="code">mem_fun1_ref</code> are provided for backwards compatibility. </p></li></ul></div><p>
  31   20.4.1 can use several different allocators; they are described on the
  32    main extensions page.
  33 </p><p>
  34   20.4.3 is extended with a special version of
  35   <code class="code">get_temporary_buffer</code> taking a second argument.  The
  36   argument is a pointer, which is ignored, but can be used to specify
  37   the template type (instead of using explicit function template
  38   arguments like the standard version does).  That is, in addition to
  39 </p><pre class="programlisting">
  40 get_temporary_buffer&lt;int&gt;(5);
  41 </pre><p>
  42 you can also use
  43 </p><pre class="programlisting">
  44 get_temporary_buffer(5, (int*)0);
  45 </pre><p>
  46   A class <code class="code">temporary_buffer</code> is given in stl_tempbuf.h.
  47 </p><p>
  48   The specialized algorithms of section 20.4.4 are extended with
  49   <code class="code">uninitialized_copy_n</code>.
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