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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_typetraits.reference.alignment_of"></a><a class="link" href="alignment_of.html" title="alignment_of">alignment_of</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">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> |
| </pre> |
| <p> |
| <span class="bold"><strong>Inherits:</strong></span> Class template <code class="computeroutput"><span class="identifier">alignment_of</span></code> |
| 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>, |
| 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 |
| alignment of type T. |
| </p> |
| <p> |
| <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 |
| a multiple of the true alignment of T, although in practice it does compute |
| the correct value in all the cases we know about.</em></span> |
| </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">alignment_of</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> |
| <span class="bold"><strong>Examples:</strong></span> |
| </p> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">alignment_of</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> |
| 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>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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 |
| with value <code class="computeroutput"><span class="identifier">ALIGNOF</span><span class="special">(</span><span class="keyword">double</span><span class="special">)</span></code>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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>. |
| </p></blockquote></div> |
| <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 note that MSVC has varying definitions of "alignment". |
| For example consider the following code: |
| </p> |
| <p> |
| </p> |
| <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> |
| <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, 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> |
| has 8-byte alignment, the final assert will fail for a 32-bit build because |
| <code class="computeroutput"><span class="identifier">a1</span></code> is not aligned on an |
| 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> |
| we would still get the same result. In fact for MSVC alignment requirements |
| (and promises) only really apply to dynamic storage, and not the stack. |
| </p> |
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