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- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_typetraits.reference.aligned_storage"></a><a class="link" href="aligned_storage.html" title="aligned_storage">aligned_storage</a>
- </h3></div></div></div>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">Size</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">Align</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">aligned_storage</span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <em class="replaceable"><code>see-below</code></em> <span class="identifier">type</span><span class="special">;</span>
- <span class="special">};</span>
- </pre>
- <p>
- <span class="bold"><strong>type:</strong></span> a built-in or POD type with size
- <code class="computeroutput"><span class="identifier">Size</span></code> and an alignment that
- is a multiple of <code class="computeroutput"><span class="identifier">Align</span></code>.
- </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">aligned_storage</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>
- <p>
- On the GCC and Visual C++ compilers (or compilers that are compatible with
- them), we support requests for types with alignments greater than any built
- in type (up to 128-bit alignment). Visual C++ users should note that such
- "extended" types can not be passed down the stack as by-value function
- arguments.
- </p>
- <div class="important"><table border="0" summary="Important">
- <tr>
- <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../doc/src/images/important.png"></td>
- <th align="left">Important</th>
- </tr>
- <tr><td align="left" valign="top">
- <p>
- Visual C++ users should be aware that MSVC has an elastic definition of
- alignment, for example consider the following code:
- </p>
- <p>
- </p>
- <pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">aligned_storage</span><span class="special"><</span><span class="number">8</span><span class="special">,</span><span class="number">8</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">align_t</span><span class="special">;</span>
- <span class="identifier">assert</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">alignment_of</span><span class="special"><</span><span class="identifier">align_t</span><span class="special">>::</span><span class="identifier">value</span> <span class="special">%</span> <span class="number">8</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
- <span class="identifier">align_t</span> <span class="identifier">a</span><span class="special">;</span>
- <span class="identifier">assert</span><span class="special">(((</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">uintptr_t</span><span class="special">)&</span><span class="identifier">a</span> <span class="special">%</span> <span class="number">8</span><span class="special">)</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
- <span class="keyword">char</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
- <span class="identifier">align_t</span> <span class="identifier">a1</span><span class="special">;</span>
- <span class="identifier">assert</span><span class="special">(((</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">uintptr_t</span><span class="special">)&</span><span class="identifier">a1</span> <span class="special">%</span> <span class="number">8</span><span class="special">)</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
- </pre>
- <p>
- </p>
- <p>
- In this code the final assert will fail for a 32-bit build because variable
- <code class="computeroutput"><span class="identifier">a1</span></code> is not aligned on an
- 8-byte boundary. Had we used the MSVC intrinsic <code class="computeroutput"><span class="identifier">__alignof</span></code>
- in place of <code class="computeroutput"><span class="identifier">alignment_of</span></code>
- or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">aligned_storage</span></code> in place of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">aligned_storage</span></code> the result would have
- been no different. In MSVC alignment requirements/promises only really
- apply to variables on the heap, not on the stack.
- </p>
- <p>
- Further, although MSVC has a mechanism for generating new types with arbitrary
- alignment requirements, such types cannot be passed as function arguments
- on the program stack. Therefore had <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">aligned_storage</span><span class="special"><</span><span class="number">8</span><span class="special">,</span><span class="number">8</span><span class="special">>::</span><span class="identifier">type</span></code>
- been a type declared with <code class="computeroutput"><span class="identifier">__declspec</span><span class="special">(</span><span class="identifier">align</span><span class="special">(</span><span class="number">8</span><span class="special">))</span></code>
- we would break a great deal of existing code that relies on being able
- to pass such types through the program stack.
- </p>
- </td></tr>
- </table></div>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <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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