source: downloads/boost_1_34_1/libs/bind/doc/ref.xml @ 33

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE library PUBLIC "-//Boost//DTD BoostBook XML V1.0//EN"
  "http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
<library name="Ref" dirname="ref" id="ref" last-revision="$Date: 2006/11/10 18:27:05 $">
  <libraryinfo>
    <author>
      <firstname>Jaakko</firstname>
      <surname>J&auml;rvi</surname>
    </author>
    <author>
      <firstname>Peter</firstname>
      <surname>Dimov</surname>
    </author>
    <author>
      <firstname>Douglas</firstname>
      <surname>Gregor</surname>
    </author>
    <author>
      <firstname>Dave</firstname>
      <surname>Abrahams</surname>
    </author>

    <copyright>
      <year>1999</year>
      <year>2000</year>
      <holder>Jaakko J&auml;rvi</holder>
    </copyright>

    <copyright>
      <year>2001</year>
      <year>2002</year>
      <holder>Peter Dimov</holder>
    </copyright>
    
    <copyright>
      <year>2002</year>
      <holder>David Abrahams</holder>
    </copyright>

    <legalnotice>
      <para>Subject to the Boost Software License, Version 1.0. See 
      accompanying file <filename>LICENSE_1_0.txt</filename> or copy at
      <ulink url="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</ulink>.
      </para>
    </legalnotice>

    <librarypurpose>A utility library for passing references to generic functions</librarypurpose>
    <librarycategory name="category:higher-order"/>
  </libraryinfo>

<title>Boost.Ref</title>

<section id="ref.intro">
  <title>Introduction</title> 

  <using-namespace name="boost"/>

  <para>The Ref library is a small library that is useful for passing
  references to function templates (algorithms) that would usually
  take copies of their arguments. It defines the class template
  <code><classname>boost::reference_wrapper&lt;T&gt;</classname></code>,
  the two functions
  <code><functionname>boost::ref</functionname></code> and
  <code><functionname>boost::cref</functionname></code> that return
  instances of <code>boost::reference_wrapper&lt;T&gt;</code>, and the
  two traits classes
  <code><classname>boost::is_reference_wrapper&lt;T&gt;</classname></code>
  and
  <code><classname>boost::unwrap_reference&lt;T&gt;</classname></code>.</para>

  <para>The purpose of
  <code>boost::reference_wrapper&lt;T&gt;</code> is to
  contain a reference to an object of type T. It is primarily used to
  "feed" references to function templates (algorithms) that take their
  parameter by value.</para>

  <para>To support this usage,
  <code>boost::reference_wrapper&lt;T&gt;</code> provides an implicit
  conversion to <code>T&amp;</code>. This usually allows the function
  templates to work on references unmodified.</para>

  <para><code>boost::reference_wrapper&lt;T&gt;</code> is
  both CopyConstructible and Assignable (ordinary references are not
  Assignable).</para>

  <para>The expression <code>boost::ref(x)</code>
  returns a
  <code>boost::reference_wrapper&lt;X&gt;(x)</code> where X
  is the type of x. Similarly,
  <code>boost::cref(x)</code> returns a
  <code>boost::reference_wrapper&lt;X const&gt;(x)</code>.</para>

  <para>The expression
  <code>boost::is_reference_wrapper&lt;T&gt;::value</code>
  is true if T is a <code>reference_wrapper</code>, and
  false otherwise.</para>

  <para>The type-expression
  <code>boost::unwrap_reference&lt;T&gt;::type</code> is T::type if T
  is a <code>reference_wrapper</code>, T otherwise.</para>
</section>

<library-reference>
<header name="boost/ref.hpp">
  <namespace name="boost">
    <class name="reference_wrapper">
      <template>
        <template-type-parameter name="T"/>
      </template>
      <purpose>
        Contains a reference to an object of type
        <computeroutput>T</computeroutput>. 
      </purpose>

      <description>
        <para><computeroutput><classname>reference_wrapper</classname></computeroutput>
        is primarily used to "feed" references to function templates
        (algorithms) that take their parameter by value.  It provides
        an implicit conversion to
        <computeroutput>T&amp;</computeroutput>, which usually allows
        the function templates to work on references
        unmodified.</para>
      </description>

      <typedef name="type"><type>T</type></typedef>

      <constructor specifiers="explicit">
        <parameter name="t">
          <paramtype>T&amp;</paramtype>
        </parameter>

        <effects><simpara>Constructs a
        <computeroutput><classname>reference_wrapper</classname></computeroutput>
        object that stores a reference to
        <computeroutput>t</computeroutput>.</simpara></effects>

        <throws><simpara>Does not throw.</simpara></throws>
      </constructor>    

      <method-group name="access">
        <method name="conversion-operator" cv="const">
          <type>T&amp;</type>
          <returns><simpara>The stored reference.</simpara></returns>
          <throws><simpara>Does not throw.</simpara></throws>
        </method>

        <method name="get" cv="const">
          <type>T&amp;</type>
          <returns><simpara>The stored reference.</simpara></returns>
          <throws><simpara>Does not throw.</simpara></throws>
        </method>

        <method name="get_pointer" cv="const">
          <type>T*</type>
          <returns><simpara>A pointer to the object referenced by the stored reference.</simpara></returns>
          <throws><simpara>Does not throw.</simpara></throws>
        </method>
      </method-group>

      <free-function-group name="constructors">
        <function name="ref">
          <type>reference_wrapper&lt;T&gt;</type>
          <parameter name="t">
            <paramtype>T&amp;</paramtype>
          </parameter>

          <returns><simpara><computeroutput><classname>reference_wrapper</classname>&lt;T&gt;(t)</computeroutput></simpara></returns>

          <throws><simpara>Does not throw.</simpara></throws>
        </function>

        <function name="cref">
          <type>reference_wrapper&lt;T const&gt;</type>
          <parameter name="t">
            <paramtype>T const&amp;</paramtype>
          </parameter>

          <returns><simpara><computeroutput><classname>reference_wrapper</classname>&lt;T const&gt;(t)</computeroutput></simpara></returns>

          <throws><simpara>Does not throw.</simpara></throws>
        </function>
      </free-function-group>
    </class>

    <class name="is_reference_wrapper">
      <template>
        <template-type-parameter name="T"/>
      </template>

      <purpose>Determine if a type <computeroutput>T</computeroutput> is an instantiation of <computeroutput><classname>reference_wrapper</classname></computeroutput>.</purpose>

      <description>
        <para>The <computeroutput>value</computeroutput> static
        constant will be <computeroutput>true</computeroutput> iff the
        type <computeroutput>T</computeroutput> is a specialization of
        <computeroutput><classname>reference_wrapper</classname></computeroutput>.</para>
      </description>

      <static-constant name="value">
        <type>bool</type>
        <default><emphasis>unspecified</emphasis></default>
      </static-constant>
    </class>

    <class name="unwrap_reference">
      <template>
        <template-type-parameter name="T"/>
      </template>
  
      <purpose>Find the type in a <computeroutput><classname>reference_wrapper</classname></computeroutput>.</purpose>

      <description>
        <para>The typedef <computeroutput>type</computeroutput> is
        <computeroutput>T::type</computeroutput> if
        <computeroutput>T</computeroutput> is a
        <computeroutput><classname>reference_wrapper</classname></computeroutput>,
        <computeroutput>T</computeroutput> otherwise.</para>
      </description>

      <typedef name="type"><type><emphasis>unspecified</emphasis></type></typedef>
    </class>
  </namespace>
</header>
</library-reference>

<section id="ref.ack">
  <title>Acknowledgements</title>

  <using-namespace name="boost"/> 

  <para><functionname>ref</functionname> and <functionname>cref</functionname>
  were originally part of the <libraryname>Tuple</libraryname> library
  by Jaakko J&auml;rvi. They were "promoted to boost:: status" by
  Peter Dimov because they are generally useful.  Douglas Gregor and
  Dave Abrahams contributed
  <classname>is_reference_wrapper</classname> and
  <classname>unwrap_reference</classname>.</para>
</section>

</library>