boost-react-native-bundle

<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Vocabulary</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../align.html" title="Chapter 2. Boost.Align"> <link rel="prev" href="reference.html" title="Reference"> <link rel="next" href="compatibility.html" title="Compatibility"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="reference.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../align.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="compatibility.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <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"> <tr> <td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td> <th align="left">Tip</th> </tr> <tr><td align="left" valign="top"> <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> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="reference.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../align.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="compatibility.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>