1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Vocabulary</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../align.html" title="Chapter 3. Boost.Align"> 10<link rel="prev" href="reference.html" title="Reference"> 11<link rel="next" href="compatibility.html" title="Compatibility"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<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> 25</div> 26<div class="section"> 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. —<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). —<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. —<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. —<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> 133<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 134<td align="left"></td> 135<td align="right"><div class="copyright-footer">Copyright © 2014-2017 Glen 136 Joseph Fernandes<p> 137 Distributed under the Boost Software License, Version 1.0. 138 </p> 139</div></td> 140</tr></table> 141<hr> 142<div class="spirit-nav"> 143<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> 144</div> 145</body> 146</html> 147
