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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="align.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
</h2></div></div></div>
<h4>
<a name="align.rationale.h0"></a>
<span class="phrase"><a name="align.rationale.dynamic_allocation"></a></span><a class="link" href="rationale.html#align.rationale.dynamic_allocation">Dynamic
allocation</a>
</h4>
<p>
C++11 added the ability to specify increased alignment (over-alignment) for
class types. Unfortunately, <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span> <span class="keyword">new</span></code>
allocation functions, <code class="computeroutput"><span class="keyword">new</span></code> expressions,
and the default allocator, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span></code>,
do not support dynamic memory allocation of over-aligned data. This library
provides allocation functions, allocators, allocator adaptors, and deleters,
that are alignment aware.
</p>
<div class="table">
<a name="align.rationale.boost_align_solutions"></a><p class="title"><b>Table 2.2. Boost.Align solutions</b></p>
<div class="table-contents"><table class="table" summary="Boost.Align solutions">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Problem
</p>
</th>
<th>
<p>
Solution
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span>
<span class="keyword">new</span><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="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">no_throw_t</span><span class="special">&)</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">aligned_alloc</span><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">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">)</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span>
<span class="keyword">delete</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">aligned_free</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">aligned_allocator</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Allocator</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">aligned_allocator_adaptor</span><span class="special"><</span><span class="identifier">Allocator</span><span class="special">></span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">default_delete</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">aligned_delete</span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h4>
<a name="align.rationale.h1"></a>
<span class="phrase"><a name="align.rationale.alignment_functions"></a></span><a class="link" href="rationale.html#align.rationale.alignment_functions">Alignment
functions</a>
</h4>
<p>
C++11 provided <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">align</span></code> in the standard library to align a
pointer value. Unfortunately some C++ standard library implementations do not
support it yet (libstdc++ as far as gcc 4.8.0) and other standard library implementations
implement it incorrectly (dinkumware in msvc 11.0). This library provides it
for those implementations and also for C++03 compilers where it is equally
useful.
</p>
<h4>
<a name="align.rationale.h2"></a>
<span class="phrase"><a name="align.rationale.alignment_traits"></a></span><a class="link" href="rationale.html#align.rationale.alignment_traits">Alignment
traits</a>
</h4>
<p>
C++11 provided the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">alignment_of</span></code> trait in the standard library
to query the alignment requirement of a type. Unfortunately some C++ standard
library vendors do not implement it in an entirely standard conforming manner,
such as for array types (libc++ as far as clang 3.4). Other vendor implementations
report incorrect values for certain types, such as pointer to members (msvc
14.0). This library provides it for those implementations and also for C++03
compilers where it is equally useful.
</p>
<h4>
<a name="align.rationale.h3"></a>
<span class="phrase"><a name="align.rationale.alignment_hints"></a></span><a class="link" href="rationale.html#align.rationale.alignment_hints">Alignment
hints</a>
</h4>
<p>
Allocating aligned memory is sometimes not enough to ensure that optimal code
is generated. Developers use specific compiler intrinsics to notify the compiler
of a given alignment property of a memory block. This library provides a macro
to abstract that functionality for compilers with the appropriate intrinsics.
</p>
<h4>
<a name="align.rationale.h4"></a>
<span class="phrase"><a name="align.rationale.alignment_testing"></a></span><a class="link" href="rationale.html#align.rationale.alignment_testing">Alignment
testing</a>
</h4>
<p>
This library provides a function to test the alignment of a pointer value.
It is generally useful in assertions to validate that memory is correctly aligned.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright © 2014 Glen Joseph Fernandes<p>
Distributed under the <a href="http://boost.org/LICENSE_1_0.txt" target="_top">Boost
Software License, Version 1.0</a>.
</p>
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