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27<a name="boost.xpressive.value"></a><div class="titlepage"></div> 28<div class="refnamediv"> 29<h2><span class="refentrytitle">Struct template value</span></h2> 30<p>boost::xpressive::value — <code class="computeroutput">value<></code> is a lazy wrapper for a value that can be used in xpressive semantic actions. </p> 31</div> 32<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2> 33<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../../xpressive/reference.html#header.boost.xpressive.xpressive_fwd_hpp" title="Header <boost/xpressive/xpressive_fwd.hpp>">boost/xpressive/xpressive_fwd.hpp</a>> 34 35</span><span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">></span> 36<span class="keyword">struct</span> <a class="link" href="value.html" title="Struct template value">value</a> <span class="special">:</span> 37 <span class="keyword">public</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">extends</span><span class="special"><</span> <span class="identifier">proto</span><span class="special">::</span><span class="identifier">terminal</span><span class="special"><</span> <span class="identifier">T</span> <span class="special">></span><span class="special">::</span><span class="identifier">type</span><span class="special">,</span> <span class="identifier">value</span><span class="special"><</span> <span class="identifier">T</span> <span class="special">></span> <span class="special">></span> 38<span class="special">{</span> 39 <span class="comment">// <a class="link" href="value.html#boost.xpressive.valueconstruct-copy-destruct">construct/copy/destruct</a></span> 40 <a class="link" href="value.html#id-1_3_47_5_18_2_1_17_6-bb"><span class="identifier">value</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span> 41 <span class="keyword">explicit</span> <a class="link" href="value.html#id-1_3_47_5_18_2_1_17_7-bb"><span class="identifier">value</span></a><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span><span class="special">)</span><span class="special">;</span> 42 43 <span class="comment">// <a class="link" href="value.html#id-1_3_47_5_18_2_1_17_5-bb">public member functions</a></span> 44 <span class="identifier">T</span> <span class="special">&</span> <a class="link" href="value.html#id-1_3_47_5_18_2_1_17_5_1-bb"><span class="identifier">get</span></a><span class="special">(</span><span class="special">)</span><span class="special">;</span> 45 <span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span> <a class="link" href="value.html#id-1_3_47_5_18_2_1_17_5_2-bb"><span class="identifier">get</span></a><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span> 46<span class="special">}</span><span class="special">;</span></pre></div> 47<div class="refsect1"> 48<a name="id-1.3.47.5.19.27.4"></a><h2>Description</h2> 49<p> 50Below is an example that shows where <code class="computeroutput"><code class="computeroutput">value<></code></code> is useful.</p> 51<pre class="programlisting"><span class="identifier">sregex</span> <span class="identifier">good_voodoo</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">shared_ptr</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">pi</span><span class="special">)</span> 52<span class="special">{</span> 53 <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">xpressive</span><span class="special">;</span> 54 <span class="comment">// Use val() to hold the shared_ptr by value:</span> 55 <span class="identifier">sregex</span> <span class="identifier">rex</span> <span class="special">=</span> <span class="special">+</span><span class="special">(</span> <span class="identifier">_d</span> <span class="special">[</span> <span class="special">++</span><span class="special">*</span><span class="identifier">val</span><span class="special">(</span><span class="identifier">pi</span><span class="special">)</span> <span class="special">]</span> <span class="special">>></span> <span class="char">'!'</span> <span class="special">)</span><span class="special">;</span> 56 <span class="comment">// OK, rex holds a reference count to the integer.</span> 57 <span class="keyword">return</span> <span class="identifier">rex</span><span class="special">;</span> 58<span class="special">}</span> 59</pre> 60<p>In the above code, <code class="computeroutput">xpressive::val()</code> is a function that returns a <code class="computeroutput">value<></code> object. Had <code class="computeroutput">val()</code> not been used here, the operation <code class="computeroutput">++*pi</code> would have been evaluated eagerly once, instead of lazily when the regex match happens. </p> 61<div class="refsect2"> 62<a name="id-1.3.47.5.19.27.4.3"></a><h3>Template Parameters</h3> 63<div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"> 64<pre class="literallayout"><span class="keyword">typename</span> T</pre> 65<p>The type of the value to store.</p> 66</li></ol></div> 67</div> 68<div class="refsect2"> 69<a name="id-1.3.47.5.19.27.4.4"></a><h3> 70<a name="boost.xpressive.valueconstruct-copy-destruct"></a><code class="computeroutput">value</code> 71 public 72 construct/copy/destruct</h3> 73<div class="orderedlist"><ol class="orderedlist" type="1"> 74<li class="listitem"> 75<pre class="literallayout"><a name="id-1_3_47_5_18_2_1_17_6-bb"></a><span class="identifier">value</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre>Store a default-constructed <code class="computeroutput">T</code>. </li> 76<li class="listitem"> 77<pre class="literallayout"><span class="keyword">explicit</span> <a name="id-1_3_47_5_18_2_1_17_7-bb"></a><span class="identifier">value</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span> t<span class="special">)</span><span class="special">;</span></pre>Store a copy of <code class="computeroutput">t</code>. <p> 78</p> 79<div class="variablelist"><table border="0" class="variablelist compact"> 80<colgroup> 81<col align="left" valign="top"> 82<col> 83</colgroup> 84<tbody><tr> 85<td><p><span class="term">Parameters:</span></p></td> 86<td><div class="variablelist"><table border="0" class="variablelist compact"> 87<colgroup> 88<col align="left" valign="top"> 89<col> 90</colgroup> 91<tbody><tr> 92<td><p><span class="term"><code class="computeroutput">t</code></span></p></td> 93<td><p>The initial value. </p></td> 94</tr></tbody> 95</table></div></td> 96</tr></tbody> 97</table></div> 98</li> 99</ol></div> 100</div> 101<div class="refsect2"> 102<a name="id-1.3.47.5.19.27.4.5"></a><h3> 103<a name="id-1_3_47_5_18_2_1_17_5-bb"></a><code class="computeroutput">value</code> public member functions</h3> 104<div class="orderedlist"><ol class="orderedlist" type="1"> 105<li class="listitem"> 106<pre class="literallayout"><span class="identifier">T</span> <span class="special">&</span> <a name="id-1_3_47_5_18_2_1_17_5_1-bb"></a><span class="identifier">get</span><span class="special">(</span><span class="special">)</span><span class="special">;</span></pre> 107<p>This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts. </p> 108</li> 109<li class="listitem"> 110<pre class="literallayout"><span class="identifier">T</span> <span class="keyword">const</span> <span class="special">&</span> <a name="id-1_3_47_5_18_2_1_17_5_2-bb"></a><span class="identifier">get</span><span class="special">(</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span></pre>Fetch the stored value. </li> 111</ol></div> 112</div> 113</div> 114</div> 115<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 116<td align="left"></td> 117<td align="right"><div class="copyright-footer">Copyright © 2007 Eric Niebler<p> 118 Distributed under the Boost Software License, Version 1.0. (See accompanying 119 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 120 </p> 121</div></td> 122</tr></table> 123<hr> 124<div class="spirit-nav"> 125<a accesskey="p" href="sub_match.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../xpressive/reference.html#header.boost.xpressive.xpressive_fwd_hpp"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../../BOOST_PROTO_FUSION_V2.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 126</div> 127</body> 128</html> 129
