boost_1_45_0/libs/proto/doc/reference/concepts/PolymorphicFunctionObject.xml

<?xml version="1.0" ?>
<concept name="PolymorphicFunctionObject" category="utility">
  <!--
  Copyright 2008 Eric Niebler

  Distributed under the Boost
  Software License, Version 1.0. (See accompanying
  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  -->
  <param name="Fn" role="polymorphic-function-object-type" />

  <models-sentence>
    The type <arg num="1" /> must be a model of <self/>.
  </models-sentence>

  <description>
    <para>
      A type that can be called and that follows the TR1 ResultOf
      protocol for return type calculation.
    </para>
  </description>

  <notation variables="fn">
    <sample-value>
      <type name="Fn" />
    </sample-value>
  </notation>

  <notation variables="a0,...an">
    <sample-value>
      <type name="A0,...An" />
    </sample-value>
  </notation>

  <associated-type name="result_type">
    <get-member-type name="type">
      <apply-template name="result_of">
        <apply-function name="Fn">
          <type name="A0,...An" />
        </apply-function>
      </apply-template>
    </get-member-type>
    <description>
      <simpara>The result of calling the Polymorphic Function Object.</simpara>
    </description>
  </associated-type>

  <valid-expression name="Function Call">
    <apply-function name="fn">
      <sample-value>
        <type name="A0,...An" />
      </sample-value>
    </apply-function>
    <return-type>
      <require-same-type testable="yes">
        <type name="result_type"/>
      </require-same-type>
    </return-type>
    <semantics>Calls the function object.</semantics>
  </valid-expression>

  <example-model>
    <type name="std::plus&lt;int&gt;" />
  </example-model>

</concept>