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  28 <h2><span class="refentrytitle">Struct template is_old_value_copyable</span></h2>
  29 <p>boost::contract::is_old_value_copyable &#8212; Trait to check if an old value type can be copied or not. </p>
  30 </div>
  31 <h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
  32 <div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../reference.html#header.boost.contract.old_hpp" title="Header &lt;boost/contract/old.hpp&gt;">boost/contract/old.hpp</a>&gt;
  33
  34 </span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> T<span class="special">&gt;</span>
  35 <span class="keyword">struct</span> <a class="link" href="is_old_value_copyable.html" title="Struct template is_old_value_copyable">is_old_value_copyable</a> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_copy_constructible</span><span class="special">&lt;</span> <span class="identifier">T</span> <span class="special">&gt;</span> <span class="special">{</span>
  36 <span class="special">}</span><span class="special">;</span></pre></div>
  37 <div class="refsect1">
  38 <a name="idm45707994778288"></a><h2>Description</h2>
  39 <p>By default, this unary boolean meta-function is equivalent to <code class="computeroutput">boost::is_copy_constructible&lt;T&gt;</code> but programmers can chose to specialize it for user-defined types (in general some kind of specialization is also needed on compilers that do not support C++11, see <a href="http://www.boost.org/doc/libs/release/libs/type_traits/doc/html/boost_typetraits/reference/is_copy_constructible.html" target="_top"><code class="computeroutput">boost::is_copy_constructible</code></a>):</p>
  40 <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">u</span><span class="special">;</span> <span class="comment">// Some user-defined type for which old values shall not be copied.</span>
  41
  42 <span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">contract</span> <span class="special">{</span>
  43     <span class="keyword">template</span><span class="special">&lt;</span><span class="special">&gt;</span> <span class="comment">// Specialization to not copy old values of type `u`.</span>
  44     <span class="keyword">struct</span> <span class="identifier">is_old_value_copyable</span><span class="special">&lt;</span><span class="identifier">u</span><span class="special">&gt;</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">false_type</span> <span class="special">{</span><span class="special">}</span><span class="special">;</span>
  45 <span class="special">}</span> <span class="special">}</span> <span class="comment">// namespace</span>
  46 </pre>
  47 <p>A given old value type <code class="computeroutput">T</code> is copied only if <code class="computeroutput">boost::contract::is_old_value_copyable&lt;T&gt;::value</code> is <code class="computeroutput">true</code>. A copyable old value type <code class="computeroutput">V</code> is always copied using <code class="computeroutput">boost::contract::old_value_copy&lt;V&gt;</code>. A non-copyable old value type <code class="computeroutput">W</code> generates a compile-time error when <code class="computeroutput">boost::contract::old_ptr&lt;W&gt;</code> is dereferenced, but instead leaves <code class="computeroutput">boost::contract::old_ptr_if_copyable&lt;W&gt;</code> always null (without generating compile-time errors).</p>
  48 <p><span class="bold"><strong>See Also:</strong></span></p>
  49 <p> <a class="link" href="../../boost_contract/extras.html#boost_contract.extras.old_value_requirements__templates_" title="Old Value Requirements (Templates)">
  50         Old Value Requirements</a> </p>
  51 <p>
  52 </p>
  53 <p>
  54 </p>
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