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  27 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  28 <a name="align.vocabulary"></a><a class="link" href="vocabulary.html" title="Vocabulary">Vocabulary</a>
  29 </h2></div></div></div>
  30 <h4>
  31 <a name="align.vocabulary.h0"></a>
  32       <span class="phrase"><a name="align.vocabulary.basic_align"></a></span><a class="link" href="vocabulary.html#align.vocabulary.basic_align">[basic.align]</a>
  33     </h4>
  34 <p>
  35       Object types have <span class="emphasis"><em>alignment requirements</em></span> which place restrictions
  36       on the addresses at which an object of that type may be allocated. An <span class="emphasis"><em>alignment</em></span>
  37       is an implementation-defined integer value representing the number of bytes
  38       between successive addresses at which a given object can be allocated. An object
  39       type imposes an alignment requirement on every object of that type; stricter
  40       alignment can be requested using the alignment specifier.
  41     </p>
  42 <p>
  43       A <span class="emphasis"><em>fundamental alignment</em></span> is represented by an alignment
  44       less than or equal to the greatest alignment supported by the implementation
  45       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. [<span class="emphasis"><em>Example:</em></span>
  48     </p>
  49 <div class="orderedlist"><ol class="orderedlist" type="1">
  50 <li class="listitem">
  51           <code class="computeroutput"><span class="keyword">struct</span> <span class="identifier">B</span>
  52           <span class="special">{</span> <span class="keyword">long</span>
  53           <span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> <span class="special">};</span></code>
  54         </li>
  55 <li class="listitem">
  56           <code class="computeroutput"><span class="keyword">struct</span> <span class="identifier">D</span>
  57           <span class="special">:</span> <span class="keyword">virtual</span>
  58           <span class="identifier">B</span> <span class="special">{</span>
  59           <span class="keyword">char</span> <span class="identifier">c</span><span class="special">;</span> <span class="special">};</span></code>
  60         </li>
  61 </ol></div>
  62 <p>
  63       When <code class="computeroutput"><span class="identifier">D</span></code> is the type of a complete
  64       object, it will have a subobject of type <code class="computeroutput"><span class="identifier">B</span></code>,
  65       so it must be aligned appropriately for a <code class="computeroutput"><span class="keyword">long</span>
  66       <span class="keyword">double</span></code>. If <code class="computeroutput"><span class="identifier">D</span></code>
  67       appears as a subobject of another object that also has <code class="computeroutput"><span class="identifier">B</span></code>
  68       as a virtual base class, the <code class="computeroutput"><span class="identifier">B</span></code>
  69       subobject might be part of a different subobject, reducing the alignment requirements
  70       on the <code class="computeroutput"><span class="identifier">D</span></code> subobject. &#8212;<span class="emphasis"><em>end
  71       example</em></span>] The result of the <code class="computeroutput"><span class="keyword">alignof</span></code>
  72       operator reflects the alignment requirement of the type in the complete-object
  73       case.
  74     </p>
  75 <p>
  76       An <span class="emphasis"><em>extended alignment</em></span> is represented by an alignment greater
  77       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
  78       extended alignments are supported and the contexts in which they are supported.
  79       A type having an extended alignment requirement is an <span class="emphasis"><em>over-aligned
  80       type</em></span>. [<span class="emphasis"><em>Note:</em></span> Every over-aligned type is or
  81       contains a class type to which extended alignment applies (possibly through
  82       a non-static data member). &#8212;<span class="emphasis"><em>end note</em></span>]
  83     </p>
  84 <p>
  85       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>.
  86       Valid alignments include only those values returned by an <code class="computeroutput"><span class="keyword">alignof</span></code>
  87       expression for the fundamental types plus an additional implementation-defined
  88       set of values, which may be empty. Every alignment value shall be a non-negative
  89       integral power of two.
  90     </p>
  91 <p>
  92       Alignments have an order from <span class="emphasis"><em>weaker</em></span> to <span class="emphasis"><em>stronger</em></span>
  93       or <span class="emphasis"><em>stricter</em></span> alignments. Stricter alignments have larger
  94       alignment values. An address that satisfies an alignment requirement also satisfies
  95       any weaker valid alignment requirement.
  96     </p>
  97 <p>
  98       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
  99       <code class="computeroutput"><span class="keyword">char</span></code>, <code class="computeroutput"><span class="keyword">signed</span>
 100       <span class="keyword">char</span></code>, and <code class="computeroutput"><span class="keyword">unsigned</span>
 101       <span class="keyword">char</span></code> shall have the weakest alignment
 102       requirement. [<span class="emphasis"><em>Note:</em></span> This enables the character types to
 103       be used as the underlying type for an aligned memory area. &#8212;<span class="emphasis"><em>end
 104       note</em></span>]
 105     </p>
 106 <p>
 107       Comparing alignments is meaningful and provides the obvious results:
 108     </p>
 109 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 110 <li class="listitem">
 111           Two alignments are equal when their numeric values are equal.
 112         </li>
 113 <li class="listitem">
 114           Two alignments are different when their numeric values are not equal.
 115         </li>
 116 <li class="listitem">
 117           When an alignment is larger than another it represents a stricter alignment.
 118         </li>
 119 </ul></div>
 120 <p>
 121       [<span class="emphasis"><em>Note:</em></span> The runtime pointer alignment function can be used
 122       to obtain an aligned pointer within a buffer; the aligned-storage templates
 123       in the library can be used to obtain aligned storage. &#8212;<span class="emphasis"><em>end
 124       note</em></span>]
 125     </p>
 126 <p>
 127       If a request for a specific extended alignment in a specific context is not
 128       supported by an implementation, the program is ill-formed. Additionally, a
 129       request for runtime allocation of dynamic storage for which the requested alignment
 130       cannot be honored shall be treated as an allocation failure.
 131     </p>
 132 </div>
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 134 <td align="left"></td>
 135 <td align="right"><div class="copyright-footer">Copyright &#169; 2014-2017 Glen
 136       Joseph Fernandes<p>
 137         Distributed under the Boost Software License, Version 1.0.
 138       </p>
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