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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="align.vocabulary"></a><a class="link" href="vocabulary.html" title="Vocabulary">Vocabulary</a> |
| </h2></div></div></div> |
| <h4> |
| <a name="align.vocabulary.h0"></a> |
| <span class="phrase"><a name="align.vocabulary.basic_align"></a></span><a class="link" href="vocabulary.html#align.vocabulary.basic_align">[basic.align]</a> |
| </h4> |
| <p> |
| Object types have <span class="bold"><strong>alignment requirements</strong></span> which |
| place restrictions on the addresses at which an object of that type may be |
| allocated. An <span class="bold"><strong>alignment</strong></span> is an implementation-defined |
| integer value representing the number of bytes between successive addresses |
| at which a given object can be allocated. An object type imposes an alignment |
| requirement on every object of that type; stricter alignment can be requested |
| using the alignment specifier. |
| </p> |
| <p> |
| A <span class="bold"><strong>fundamental alignment</strong></span> is represented by |
| an alignment less than or equal to the greatest alignment supported by the |
| implementation in all contexts, which is equal to <code class="computeroutput"><span class="keyword">alignof</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">max_align_t</span><span class="special">)</span></code>. |
| The alignment required for a type might be different when it is used as the |
| type of a complete object and when it is used as the type of a subobject. |
| </p> |
| <div class="tip"><table border="0" summary="Tip"> |
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| <th align="left">Tip</th> |
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| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{</span> <span class="keyword">long</span> <span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> <span class="special">};</span> |
| <span class="keyword">struct</span> <span class="identifier">D</span> <span class="special">:</span> <span class="keyword">virtual</span> <span class="identifier">B</span> <span class="special">{</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">;</span> <span class="special">};</span> |
| </pre> |
| <p> |
| When <code class="computeroutput"><span class="identifier">D</span></code> is the type of a complete |
| object, it will have a subobject of type <code class="computeroutput"><span class="identifier">B</span></code>, |
| so it must be aligned appropriately for a <code class="computeroutput"><span class="keyword">long</span> |
| <span class="keyword">double</span></code>. If <code class="computeroutput"><span class="identifier">D</span></code> |
| appears as a subobject of another object that also has <code class="computeroutput"><span class="identifier">B</span></code> |
| as a virtual base class, the <code class="computeroutput"><span class="identifier">B</span></code> |
| subobject might be part of a different subobject, reducing the alignment |
| requirements on the <code class="computeroutput"><span class="identifier">D</span></code> subobject. |
| </p> |
| </td></tr> |
| </table></div> |
| <p> |
| The result of the <code class="computeroutput"><span class="keyword">alignof</span></code> operator |
| reflects the alignment requirement of the type in the complete-object case. |
| </p> |
| <p> |
| An <span class="bold"><strong>extended alignment</strong></span> is represented by an |
| alignment greater than <code class="computeroutput"><span class="keyword">alignof</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">max_align_t</span><span class="special">)</span></code>. |
| It is implementation-defined whether any extended alignments are supported |
| and the contexts in which they are supported. A type having an extended alignment |
| requirement is an <span class="bold"><strong>over-aligned type</strong></span>. |
| </p> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Every over-aligned type is or contains a class type to which extended alignment |
| applies (possibly through a non-static data member). |
| </p></td></tr> |
| </table></div> |
| <p> |
| Alignments are represented as values of the type <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>. |
| Valid alignments include only those values returned by an <code class="computeroutput"><span class="keyword">alignof</span></code> |
| expression for the fundamental types plus an additional implementation-defined |
| set of values, which may be empty. Every alignment value shall be a non-negative |
| integral power of two. |
| </p> |
| <p> |
| Alignments have an order from <span class="bold"><strong>weaker</strong></span> to <span class="bold"><strong>stronger</strong></span> or <span class="bold"><strong>stricter</strong></span> |
| alignments. Stricter alignments have larger alignment values. An address that |
| satisfies an alignment requirement also satisfies any weaker valid alignment |
| requirement. |
| </p> |
| <p> |
| The alignment requirement of a complete type can be queried using an <code class="computeroutput"><span class="keyword">alignof</span></code> expression. Furthermore, the types |
| <code class="computeroutput"><span class="keyword">char</span></code>, <code class="computeroutput"><span class="keyword">signed</span> |
| <span class="keyword">char</span></code>, and <code class="computeroutput"><span class="keyword">unsigned</span> |
| <span class="keyword">char</span></code> shall have the weakest alignment |
| requirement. |
| </p> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| This enables the character types to be used as the underlying type for an |
| aligned memory area. |
| </p></td></tr> |
| </table></div> |
| <p> |
| Comparing alignments is meaningful and provides the obvious results: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| Two alignments are equal when their numeric values are equal. |
| </li> |
| <li class="listitem"> |
| Two alignments are different when their numeric values are not equal. |
| </li> |
| <li class="listitem"> |
| When an alignment is larger than another it represents a stricter alignment. |
| </li> |
| </ul></div> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| The runtime pointer alignment function can be used to obtain an aligned pointer |
| within a buffer; the aligned-storage templates in the library can be used |
| to obtain aligned storage. |
| </p></td></tr> |
| </table></div> |
| <p> |
| If a request for a specific extended alignment in a specific context is not |
| supported by an implementation, the program is ill-formed. Additionally, a |
| request for runtime allocation of dynamic storage for which the requested alignment |
| cannot be honored shall be treated as an allocation failure. |
| </p> |
| </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 © 2014 Glen Fernandes<p> |
| Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost |
| Software License, Version 1.0</a>. |
| </p> |
| </div></td> |
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