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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="boost_typetraits.reference.alignment_of"></a><a class="link" href="alignment_of.html" title="alignment_of">alignment_of</a> | |
28 | </h3></div></div></div> | |
29 | <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> | |
30 | <span class="keyword">struct</span> <span class="identifier">alignment_of</span> <span class="special">:</span> <span class="keyword">public</span> <a class="link" href="integral_constant.html" title="integral_constant">integral_constant</a><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="identifier">ALIGNOF</span><span class="special">(</span><span class="identifier">T</span><span class="special">)></span> <span class="special">{};</span> | |
31 | </pre> | |
32 | <p> | |
33 | <span class="bold"><strong>Inherits:</strong></span> Class template <code class="computeroutput"><span class="identifier">alignment_of</span></code> | |
34 | inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">integral_constant</a><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="identifier">ALIGNOF</span><span class="special">(</span><span class="identifier">T</span><span class="special">)></span></code>, | |
35 | where <code class="computeroutput"><span class="identifier">ALIGNOF</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span></code> is the | |
36 | alignment of type T. | |
37 | </p> | |
38 | <p> | |
39 | <span class="emphasis"><em>Note: strictly speaking you should only rely on the value of <code class="computeroutput"><span class="identifier">ALIGNOF</span><span class="special">(</span><span class="identifier">T</span><span class="special">)</span></code> being | |
40 | a multiple of the true alignment of T, although in practice it does compute | |
41 | the correct value in all the cases we know about.</em></span> | |
42 | </p> | |
43 | <p> | |
44 | <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span> | |
45 | <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">alignment_of</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> | |
46 | 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> | |
47 | </p> | |
48 | <p> | |
49 | <span class="bold"><strong>Examples:</strong></span> | |
50 | </p> | |
51 | <div class="blockquote"><blockquote class="blockquote"><p> | |
52 | <code class="computeroutput"><span class="identifier">alignment_of</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> | |
53 | inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">integral_constant</a><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="identifier">ALIGNOF</span><span class="special">(</span><span class="keyword">int</span><span class="special">)></span></code>. | |
54 | </p></blockquote></div> | |
55 | <div class="blockquote"><blockquote class="blockquote"><p> | |
56 | <code class="computeroutput"><span class="identifier">alignment_of</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">type</span></code> is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">integral_constant</a><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="identifier">ALIGNOF</span><span class="special">(</span><span class="keyword">char</span><span class="special">)></span></code>. | |
57 | </p></blockquote></div> | |
58 | <div class="blockquote"><blockquote class="blockquote"><p> | |
59 | <code class="computeroutput"><span class="identifier">alignment_of</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">value</span></code> is an integral constant expression | |
60 | with value <code class="computeroutput"><span class="identifier">ALIGNOF</span><span class="special">(</span><span class="keyword">double</span><span class="special">)</span></code>. | |
61 | </p></blockquote></div> | |
62 | <div class="blockquote"><blockquote class="blockquote"><p> | |
63 | <code class="computeroutput"><span class="identifier">alignment_of</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>. | |
64 | </p></blockquote></div> | |
65 | <div class="important"><table border="0" summary="Important"> | |
66 | <tr> | |
67 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../../../../doc/src/images/important.png"></td> | |
68 | <th align="left">Important</th> | |
69 | </tr> | |
70 | <tr><td align="left" valign="top"> | |
71 | <p> | |
72 | Visual C++ users should note that MSVC has varying definitions of "alignment". | |
73 | For example consider the following code: | |
74 | </p> | |
75 | <p> | |
76 | </p> | |
77 | <pre class="programlisting"><span class="keyword">typedef</span> <span class="keyword">long</span> <span class="keyword">long</span> <span class="identifier">align_t</span><span class="special">;</span> | |
78 | <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> | |
79 | <span class="identifier">align_t</span> <span class="identifier">a</span><span class="special">;</span> | |
80 | <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> | |
81 | <span class="keyword">char</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> | |
82 | <span class="identifier">align_t</span> <span class="identifier">a1</span><span class="special">;</span> | |
83 | <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> | |
84 | </pre> | |
85 | <p> | |
86 | </p> | |
87 | <p> | |
88 | In this code, even though <code class="computeroutput"><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></code> reports that <code class="computeroutput"><span class="identifier">align_t</span></code> | |
89 | has 8-byte alignment, the final assert will fail for a 32-bit build because | |
90 | <code class="computeroutput"><span class="identifier">a1</span></code> is not aligned on an | |
91 | 8 byte boundary. Note that had we used the MSVC intrinsic <code class="computeroutput"><span class="identifier">__alignof</span></code> in place of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">alignment_of</span></code> | |
92 | we would still get the same result. In fact for MSVC alignment requirements | |
93 | (and promises) only really apply to dynamic storage, and not the stack. | |
94 | </p> | |
95 | </td></tr> | |
96 | </table></div> | |
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98 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
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100 | <td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, | |
101 | Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, | |
102 | Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten | |
103 | Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven | |
104 | Watanabe<p> | |
105 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
106 | 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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