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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="emphasis"><em>alignment requirements</em></span> which place restrictions
on the addresses at which an object of that type may be allocated. An <span class="emphasis"><em>alignment</em></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="emphasis"><em>fundamental alignment</em></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. [<span class="emphasis"><em>Example:</em></span>
</p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
<code class="computeroutput"><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></code>
</li>
<li class="listitem">
<code class="computeroutput"><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></code>
</li>
</ol></div>
<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. —<span class="emphasis"><em>end
example</em></span>] 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="emphasis"><em>extended alignment</em></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="emphasis"><em>over-aligned
type</em></span>. [<span class="emphasis"><em>Note:</em></span> Every over-aligned type is or
contains a class type to which extended alignment applies (possibly through
a non-static data member). —<span class="emphasis"><em>end note</em></span>]
</p>
<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="emphasis"><em>weaker</em></span> to <span class="emphasis"><em>stronger</em></span>
or <span class="emphasis"><em>stricter</em></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. [<span class="emphasis"><em>Note:</em></span> This enables the character types to
be used as the underlying type for an aligned memory area. —<span class="emphasis"><em>end
note</em></span>]
</p>
<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>
<p>
[<span class="emphasis"><em>Note:</em></span> 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. —<span class="emphasis"><em>end
note</em></span>]
</p>
<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>
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<td align="right"><div class="copyright-footer">Copyright © 2014-2017 Glen
Joseph Fernandes<p>
Distributed under the Boost Software License, Version 1.0.
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