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  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="align.vocabulary"></a><a class="link" href="vocabulary.html" title="Vocabulary">Vocabulary</a>
  28 </h2></div></div></div>
  29 <h4>
  30 <a name="align.vocabulary.h0"></a>
  31       <span class="phrase"><a name="align.vocabulary.basic_align"></a></span><a class="link" href="vocabulary.html#align.vocabulary.basic_align">[basic.align]</a>
  32     </h4>
  33 <p>
  34       Object types have <span class="bold"><strong>alignment requirements</strong></span> which
  35       place restrictions on the addresses at which an object of that type may be
  36       allocated. An <span class="bold"><strong>alignment</strong></span> is an implementation-defined
  37       integer value representing the number of bytes between successive addresses
  38       at which a given object can be allocated. An object type imposes an alignment
  39       requirement on every object of that type; stricter alignment can be requested
  40       using the alignment specifier.
  41     </p>
  42 <p>
  43       A <span class="bold"><strong>fundamental alignment</strong></span> is represented by
  44       an alignment less than or equal to the greatest alignment supported by the
  45       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>.
  46       The alignment required for a type might be different when it is used as the
  47       type of a complete object and when it is used as the type of a subobject.
  48     </p>
  49 <div class="tip"><table border="0" summary="Tip">
  50 <tr>
  51 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td>
  52 <th align="left">Tip</th>
  53 </tr>
  54 <tr><td align="left" valign="top">
  55 <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>
  56 <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>
  57 </pre>
  58 <p>
  59         When <code class="computeroutput"><span class="identifier">D</span></code> is the type of a complete
  60         object, it will have a subobject of type <code class="computeroutput"><span class="identifier">B</span></code>,
  61         so it must be aligned appropriately for a <code class="computeroutput"><span class="keyword">long</span>
  62         <span class="keyword">double</span></code>. If <code class="computeroutput"><span class="identifier">D</span></code>
  63         appears as a subobject of another object that also has <code class="computeroutput"><span class="identifier">B</span></code>
  64         as a virtual base class, the <code class="computeroutput"><span class="identifier">B</span></code>
  65         subobject might be part of a different subobject, reducing the alignment
  66         requirements on the <code class="computeroutput"><span class="identifier">D</span></code> subobject.
  67       </p>
  68 </td></tr>
  69 </table></div>
  70 <p>
  71       The result of the <code class="computeroutput"><span class="keyword">alignof</span></code> operator
  72       reflects the alignment requirement of the type in the complete-object case.
  73     </p>
  74 <p>
  75       An <span class="bold"><strong>extended alignment</strong></span> is represented by an
  76       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>.
  77       It is implementation-defined whether any extended alignments are supported
  78       and the contexts in which they are supported. A type having an extended alignment
  79       requirement is an <span class="bold"><strong>over-aligned type</strong></span>.
  80     </p>
  81 <div class="note"><table border="0" summary="Note">
  82 <tr>
  83 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
  84 <th align="left">Note</th>
  85 </tr>
  86 <tr><td align="left" valign="top"><p>
  87         Every over-aligned type is or contains a class type to which extended alignment
  88         applies (possibly through a non-static data member).
  89       </p></td></tr>
  90 </table></div>
  91 <p>
  92       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>.
  93       Valid alignments include only those values returned by an <code class="computeroutput"><span class="keyword">alignof</span></code>
  94       expression for the fundamental types plus an additional implementation-defined
  95       set of values, which may be empty. Every alignment value shall be a non-negative
  96       integral power of two.
  97     </p>
  98 <p>
  99       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>
 100       alignments. Stricter alignments have larger alignment values. An address that
 101       satisfies an alignment requirement also satisfies any weaker valid alignment
 102       requirement.
 103     </p>
 104 <p>
 105       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
 106       <code class="computeroutput"><span class="keyword">char</span></code>, <code class="computeroutput"><span class="keyword">signed</span>
 107       <span class="keyword">char</span></code>, and <code class="computeroutput"><span class="keyword">unsigned</span>
 108       <span class="keyword">char</span></code> shall have the weakest alignment
 109       requirement.
 110     </p>
 111 <div class="note"><table border="0" summary="Note">
 112 <tr>
 113 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 114 <th align="left">Note</th>
 115 </tr>
 116 <tr><td align="left" valign="top"><p>
 117         This enables the character types to be used as the underlying type for an
 118         aligned memory area.
 119       </p></td></tr>
 120 </table></div>
 121 <p>
 122       Comparing alignments is meaningful and provides the obvious results:
 123     </p>
 124 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 125 <li class="listitem">
 126           Two alignments are equal when their numeric values are equal.
 127         </li>
 128 <li class="listitem">
 129           Two alignments are different when their numeric values are not equal.
 130         </li>
 131 <li class="listitem">
 132           When an alignment is larger than another it represents a stricter alignment.
 133         </li>
 134 </ul></div>
 135 <div class="note"><table border="0" summary="Note">
 136 <tr>
 137 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../doc/src/images/note.png"></td>
 138 <th align="left">Note</th>
 139 </tr>
 140 <tr><td align="left" valign="top"><p>
 141         The runtime pointer alignment function can be used to obtain an aligned pointer
 142         within a buffer; the aligned-storage templates in the library can be used
 143         to obtain aligned storage.
 144       </p></td></tr>
 145 </table></div>
 146 <p>
 147       If a request for a specific extended alignment in a specific context is not
 148       supported by an implementation, the program is ill-formed. Additionally, a
 149       request for runtime allocation of dynamic storage for which the requested alignment
 150       cannot be honored shall be treated as an allocation failure.
 151     </p>
 152 </div>
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 154 <td align="left"></td>
 155 <td align="right"><div class="copyright-footer">Copyright &#169; 2014 Glen Fernandes<p>
 156         Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost
 157         Software License, Version 1.0</a>.
 158       </p>
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