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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Boost: ref.hpp documentation</title>
</head>
<body bgcolor="White">
<table border="0" width="100%">
<tr>
<td width="277">
<img src="../../c++boost.gif" alt="c++boost.gif (8819 bytes)" width="277" height="86">
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<td align="center">
<table border="0">
<tr><td nowrap><h1>ref.hpp</h1></td></tr>
<tr><td align="right" nowrap><small> 1.00.0004 (2002-01-27)</small></td></tr>
</table>
</td>
</tr>
<tr>
<td colspan="2" height="64"> </td>
</tr>
</table>
<h2>Files</h2>
<ul>
<li><a href="../../boost/ref.hpp">ref.hpp</a>
</ul>
<h2>Purpose</h2>
<p>
The header <a href="../../boost/ref.hpp">boost/ref.hpp</a> defines the class template
<b>boost::reference_wrapper<T></b>, the two functions <b>boost::ref</b> and
<b>boost::cref</b> that return instances of
<b>boost::reference_wrapper<T></b>, and the two traits classes <b>boost::is_reference_wrapper<T></b> and <b>boost::unwrap_reference<T></b>.
</p>
<p>
The purpose of <b>boost::reference_wrapper<T></b> is to contain a reference to
an object of type <b>T</b>. It is primarily used to "feed" references to
function templates (algorithms) that take their parameter by value.
</p>
<p>
To support this usage, <b>boost::reference_wrapper<T></b> provides an implicit
conversion to <b>T &</b>. This usually allows the function templates to
work on references unmodified.
</p>
<p>
<b>boost::reference_wrapper<T></b> is both <b>CopyConstructible</b> and
<b>Assignable</b> (ordinary references are not <b>Assignable</b>).
</p>
<p>
The expression <b>boost::ref(x)</b> returns a <b>boost::reference_wrapper<X>(x)</b>
where <b>X</b> is the type of <b>x</b>. Similarly, <b>boost::cref(x)</b>
returns a <b>boost::reference_wrapper<X const>(x)</b>.
</p>
<p>The expression <b>boost::is_reference_wrapper<T>::value</b> is
<b>true</b> if <b>T</b> is a
<b>reference_wrapper</b>, and <b>false</b> otherwise.
<p>The type-expression <b>boost::unwrap_reference<T>::type</b>
is <b>T::type</b> if <b>T</b> is a
<b>reference_wrapper</b>, <b>T</b> otherwise.
<h2>Interface</h2>
<h3>Synopsis</h3>
<pre>
namespace boost
{
template<class T> class <a href="#reference_wrapper">reference_wrapper</a>;
template<class T> reference_wrapper<T> <a href="#ref">ref</a>(T & t);
template<class T> reference_wrapper<T const> <a href="#cref">cref</a>(T const & t);
template<class T> class is_reference_wrapper<T const>;
template<class T> class unwrap_reference<T const>;
}
</pre>
<h3><a name="reference_wrapper">reference_wrapper</a></h3>
<pre>
template<class T> class reference_wrapper
{
public:
typedef T type;
explicit <a href="#rt_construct">reference_wrapper</a>(T & t);
<a href="#rt_operator">operator T &</a> () const;
T & <a href="#rt_get">get</a>() const;
};
</pre>
<h4><a name="rt_construct">explicit reference_wrapper(T & t)</a></h4>
<blockquote>
<p>
<b>Effects:</b> Constructs a <b>reference_wrapper</b> object that stores a reference to <b>t</b>.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h4><a name="rt_operator">operator T & () const</a></h4>
<blockquote>
<p>
<b>Returns:</b> the stored reference.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h4><a name="rt_get">T & get() const</a></h4>
<blockquote>
<p>
<b>Returns:</b> the stored reference.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h3><a name="ref">ref</a></h3>
<pre>
template<class T> reference_wrapper<T> ref(T & t);
</pre>
<blockquote>
<p>
<b>Returns:</b> <tt>reference_wrapper<T>(t)</tt>.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h3><a name="cref">cref</a></h3>
<pre>
template<class T> reference_wrapper<T const> cref(T const & t);
</pre>
<blockquote>
<p>
<b>Returns:</b> <tt>reference_wrapper<T const>(t)</tt>.
</p>
<p>
<b>Throws:</b> Nothing.
</p>
</blockquote>
<h3><a name="is_reference_wrapper">is_reference_wrapper</a></h3>
<pre>
template<class T> class is_reference_wrapper<T const>
{
public:
static bool value = <i>unspecified</i>;
};
</pre>
Value is <b>true</b> iff <tt>T</tt> is a specialization of <tt>reference_wrapper</tt>.
<h3><a name="unwrap_reference">unwrap_reference</a></h3>
<pre>
template<class T> class unwrap_reference<T const>
{
public:
typedef <i>unspecified</i> type;
};
</pre>
<tt>type</tt> is equivalent to <tt>T::type</tt> if <tt>T</tt> is a specialization of <tt>reference_wrapper</tt>. Otherwise <tt>type</tt> is equivalent to <tt>T</tt>.
<h2>Acknowledgements</h2>
<p>
<b>ref</b> and <b>cref</b> were originally part of the Boost.Tuple
library by <a href="../../people/jaakko_jarvi.htm"> Jaakko
Järvi</a>. They were "promoted to <b>boost::</b> status" by <a
href="../../people/peter_dimov.htm">Peter Dimov</a> because they are
generally useful. <a href="../../people/doug_gregor.html">Douglas
Gregor</a> and <a href="../../people/dave_abrahams.htm">Dave
Abrahams</a> contributed <tt>is_reference_wrapper</tt> and
<tt>unwrap_reference</tt>.
</p>
<p><br><br><br><small>Copyright © 2001 by Peter Dimov and Multi Media
Ltd. Permission to copy, use, modify, sell and distribute this document is
granted provided this copyright notice appears in all copies. This document
is provided "as is" without express or implied warranty, and with
no claim as to its suitability for any purpose.</small></p>
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