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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_typetraits.reference.is_copy_assignable"></a><a class="link" href="is_copy_assignable.html" title="is_copy_assignable">is_copy_assignable</a> |
| </h3></div></div></div> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">is_copy_assignable</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span> |
| </pre> |
| <p> |
| <span class="bold"><strong>Inherits:</strong></span> If <code class="computeroutput"><span class="identifier">T</span></code> |
| is <code class="computeroutput"><span class="identifier">CopyAssignable</span></code> (i.e. has |
| an accessible explicit or implicit copy assignment operator), then inherits |
| from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>, |
| otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>. |
| Type <code class="computeroutput"><span class="identifier">T</span></code> must be a complete |
| type. |
| </p> |
| <p> |
| In other words, inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a> |
| only if copy assignment of <code class="computeroutput"><span class="identifier">T</span></code> |
| from <code class="computeroutput"><span class="keyword">const</span> <span class="identifier">T</span> |
| <span class="special">&</span></code> is not marked with <code class="computeroutput"><span class="special">=</span> <span class="keyword">delete</span></code>, |
| <code class="computeroutput"><span class="identifier">T</span></code> does not derives from |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span></code> and is not marked with <code class="computeroutput"><span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span></code>. |
| </p> |
| <p> |
| <span class="bold"><strong>Compiler Compatibility:</strong></span> If the compiler |
| does not support partial-specialization of class templates, then this template |
| can not be used. |
| </p> |
| <p> |
| If your compiler does not support C++11 deleted functions (<code class="computeroutput"><span class="special">=</span> <span class="keyword">delete</span></code>) |
| or does not support SFINAE for the deleted assignments, then derive your |
| classes from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span></code> or mark them with <code class="computeroutput"><span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span></code> to show |
| that class is non-assignable. |
| </p> |
| <p> |
| Trait does not care about access modifiers, so if you see errors like this: |
| </p> |
| <pre class="programlisting"><span class="char">'T::operator=(const T&)'</span> <span class="identifier">is</span> <span class="keyword">private</span> |
| <span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_copy_assignable</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">:</span><span class="number">68</span><span class="special">:</span><span class="number">5</span><span class="special">:</span> <span class="identifier">error</span><span class="special">:</span> <span class="identifier">within</span> <span class="keyword">this</span> <span class="identifier">context</span> |
| </pre> |
| <p> |
| then you are trying to call that macro for a structure with private assignment: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">T</span> <span class="special">{</span> |
| <span class="comment">// ...</span> |
| <span class="keyword">private</span><span class="special">:</span> |
| <span class="identifier">T</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&);</span> |
| <span class="comment">// ...</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| To fix that you must modify your structure, explicitly marking it as noncopyable |
| (<code class="computeroutput"><span class="special">=</span> <span class="keyword">delete</span></code>, |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">noncopyable</span></code> or <code class="computeroutput"><span class="identifier">BOOST_MOVABLE_BUT_NOT_COPYABLE</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span></code>) |
| or explicitly <a class="link" href="../user_defined.html" title="User Defined Specializations">specializing |
| the trait</a>. |
| </p> |
| <p> |
| <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span> |
| <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_copy_assignable</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> |
| or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> |
| </p> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, |
| Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, |
| Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten |
| Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven |
| Watanabe<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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>) |
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