ceph/src/boost/libs/type_traits/doc/html/boost_typetraits/reference/alignment_of.html

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  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">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</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">&lt;</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">)&gt;</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">&lt;</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">)&gt;</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">&lt;</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">&gt;</span></code>
  46         or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</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">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
  53           inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">integral_constant</a><span class="special">&lt;</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">)&gt;</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">&lt;</span><span class="keyword">char</span><span class="special">&gt;::</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">&lt;</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">)&gt;</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">&lt;</span><span class="keyword">double</span><span class="special">&gt;::</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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">&lt;</span><span class="identifier">align_t</span><span class="special">&gt;::</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">)&amp;</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">)&amp;</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">&lt;</span><span class="identifier">align_t</span><span class="special">&gt;</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>
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