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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> |
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| <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> |
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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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