239 lines
26 KiB
HTML
239 lines
26 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>对象与对齐</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_language_object skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading">对象与对齐</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-language.html">language</a></span></div>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<p>C 程序创建、销毁、访问并操作对象。</p>
|
|
<p>C 中，一个对象是执行环境中<a href="c-language-memory_model.html">数据存储</a>的一个区域，其内容可以表示<i>值</i>（值是对象的内容转译为特定<a href="c-language-types.html">类型</a>时的含义）。</p>
|
|
<p>每个对象拥有</p>
|
|
<ul>
|
|
<li>大小（可由 <a href="c-language-sizeof.html"><code>sizeof</code></a> 确定）</li>
|
|
<li>对齐要求<span class="t-rev-inl t-since-c11"><span>（可由 <a href="c-language-_Alignof.html"><code>_Alignof</code></a> 确定）</span> <span><span class="t-mark-rev t-since-c11">(C11 起)</span></span></span></li>
|
|
<li><a href="c-language-storage_duration.html">存储期</a>（自动、静态、分配、线程局域）</li>
|
|
<li><a href="c-language-lifetime.html">生存期</a>（等于存储期或临时）</li>
|
|
<li>有效类型（见下）</li>
|
|
<li>值（可以是不确定的）</li>
|
|
<li>可选项，表示该对象的<a href="c-language-identifier.html">标识符</a></li>
|
|
</ul>
|
|
<p>对象由<a href="c-language-declarations.html">声明</a>、<a href="c-memory.html">分配函数</a>、<a href="c-language-string_literal.html">字符串字面量</a>、<a href="c-language-compound_literal.html">复合字面量</a>，及返回<a href="c-language-lifetime.html">拥有数组类型的结构体或联合体</a>的非左值表达式创建。</p>
|
|
<h3><span class="mw-headline" id=".E5.AF.B9.E8.B1.A1.E8.A1.A8.E7.A4.BA">对象表示</span></h3>
|
|
<p>除了<a href="c-language-bit_field.html">位域</a>，每个对象都是由一个或更多字节组成的，每个字节由 <span class="t-lc"><a href="c-types-limits.html">CHAR_BIT</a></span> 位组成，而且每个对象可以用 <span class="t-lc"><a href="c-string-byte-memcpy.html">memcpy</a></span> 复制到 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">char</span><span class="br0">[</span>n<span class="br0">]</span></span></span> 类型的对象中，这里 <code>n</code> 是对象的大小。生成的数组内容被称为<i>对象表示</i>。</p>
|
|
<p>若两对象拥有相同的对象表示，则它们比较相等（除了它们是浮点数NaN的情况）。逆命题非真：两个比较相等的对象可以拥有不同的对象表示，因为并非对象表示的每一位都需要参与其值。这些位可以用于填充以满足对齐要求，等同性检测，指示陷阱表示等。</p>
|
|
<p>若一个对象表示不表示该对象类型的任何值，则它被称为<i>陷阱表示</i>。以异于字符类型左值表达式读取的方式访问陷阱表示是未定义行为。结构体或联合体的值始终不是陷阱表示，即使任何一个成员的值是陷阱表示。</p>
|
|
<p>对于 <code>char</code> 、 <code>signed char</code> 、 <code>unsigned char</code> 类型的对象，对象表示的每一位都要求参与其值表示，而且每种可能的位模式都表示不同的值（不允许填充位、陷阱位或多重表示）。</p>
|
|
<p><a href="c-language-arithmetic_types.html#.E6.95.B4.E6.95.B0.E7.B1.BB.E5.9E.8B">整数类型</a>（ short 、 int 、 long 、 long long ）对象占用多个字节时，这些字节的用法是实现定义的，不过二种有主导地位的实现是<i>大端 (big-endian)</i> （ POWER 、 Sparc 、 Itanium ）和<i>小端 (littel-endian)</i> （ x86 、 x86-64 ）：大端平台将最高位字节存储于整数所占据的存储区域的最低地址，小端平台将最低位字节存储于最低地址。细节见<a href="https://en.wikipedia.org/wiki/Endianness" class="extiw" title="enwiki:Endianness">端序</a>。参阅下方示例。</p>
|
|
<p>尽管大多数实现都不允许整数的陷阱表示、填充位或多重表示，也还存在例外；例如 Itanium 上的整数类型值<a rel="nofollow" class="external text" href="https://blogs.msdn.microsoft.com/oldnewthing/20040119-00/?p=41003">就可以是陷阱表示</a>。</p>
|
|
<h3><span class="mw-headline" id=".E6.9C.89.E6.95.88.E7.B1.BB.E5.9E.8B">有效类型</span></h3>
|
|
<p>每个对象都拥有<i>有效类型</i>，它决定何种<a href="c-language-value_category.html">左值</a>访问合法，何种违反严格别名使用规则。</p>
|
|
<p>若对象是由<a href="c-language-declarations.html">声明</a>创建的，则该对象的声明类型即是对象的<i>有效类型</i>。</p>
|
|
<p>若对象由<a href="c-memory.html">分配函数</a>（包含 <span class="t-lc"><a href="c-memory-realloc.html">realloc</a></span> ）创建，则它没有声明类型。这种对象以下列方式获得有效类型：</p>
|
|
<ul>
|
|
<li>首次通过拥有异于字符类型的类型的左值写入该对象，无论何时该左值的类型都会成为该对象该次写入和所有后继读取的<i>有效类型</i>。</li>
|
|
<li><span class="t-lc"><a href="c-string-byte-memcpy.html">memcpy</a></span> 或 <span class="t-lc"><a href="c-string-byte-memmove.html">memmove</a></span> 复制另一个对象到该对象，无论何时源对象的有效类型（若它有）都会成为该对象该次写入和所有后继读取的<i>有效类型</i>。</li>
|
|
<li>任何其他对无声明类型的对象的访问，有效类型是访问所用的左值类型。</li>
|
|
</ul>
|
|
<h3><span class="mw-headline" id=".E4.B8.A5.E6.A0.BC.E5.88.AB.E5.90.8D.E4.BD.BF.E7.94.A8">严格别名使用</span></h3>
|
|
<p>给定一个拥有<i>有效类型</i> <code>T1</code> 的对象，使用相异类型的 <code>T2</code> 左值表达式（典型的是解引用指针）访问它是未定义行为，除非：</p>
|
|
<ul>
|
|
<li><code>T2</code> 和 <code>T1</code> 是<a href="c-language-types.html#.E5.85.BC.E5.AE.B9.E7.B1.BB.E5.9E.8B">兼容类型</a>。</li>
|
|
<li><code>T2</code> 是与 <code>T1</code> <a href="c-language-types.html#.E5.85.BC.E5.AE.B9.E7.B1.BB.E5.9E.8B">兼容</a>的类型的 cvr 限定版本。</li>
|
|
<li><code>T2</code> 是与 <code>T1</code> <a href="c-language-types.html#.E5.85.BC.E5.AE.B9.E7.B1.BB.E5.9E.8B">兼容</a>的类型的有符号或无符号版本。</li>
|
|
<li><code>T2</code> 是聚合体或联合体类型，其成员中包含一个前述类型（递归地包括子聚合体或被包含联合体的成员）。</li>
|
|
<li><code>T2</code> 是字符类型（ <code>char</code> 、 <code>signed char</code> 或 <code>unsigned char</code> ）。</li>
|
|
</ul>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="c source-c">
|
|
<pre class="de1"><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span>
|
|
<span class="kw4">char</span><span class="sy2">*</span> pc <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="sy3">&</span>i<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw1">if</span><span class="br0">(</span>pc<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span> <span class="sy1">==</span> <span class="st0">'\x7'</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// 通过 char 别名使用是 OK 的</span>
|
|
<a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"This system is little-endian"</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span>
|
|
<a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"This system is big-endian"</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">float</span><span class="sy2">*</span> pf <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">float</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="sy3">&</span>i<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw4">float</span> d <span class="sy1">=</span> <span class="sy2">*</span>pf<span class="sy4">;</span> <span class="co1">// UB ： float 左值 *p 不能用来访问 int</span></pre></div>
|
|
</div>
|
|
<p>这些规则控制接受二个指针的函数，在通过一个指针写入后，是否必须重读取另一个：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="c source-c">
|
|
<pre class="de1"><span class="co1">// int* 与 double* 不能别名使用</span>
|
|
<span class="kw4">void</span> f1<span class="br0">(</span><span class="kw4">int</span> <span class="sy2">*</span>pi, <span class="kw4">double</span> <span class="sy2">*</span>pd, <span class="kw4">double</span> d<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="co1">// 从 *pi 的读取可以只做一次，在循环前</span>
|
|
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> <span class="sy2">*</span>pi<span class="sy4">;</span> i<span class="sy2">++</span><span class="br0">)</span> <span class="sy2">*</span>pd<span class="sy2">++</span> <span class="sy1">=</span> d<span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="c source-c">
|
|
<pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span> <span class="kw4">int</span> a, b<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="co1">// int* 和 struct S* 可以别名使用，因为 S 拥有 int 类型的成员</span>
|
|
<span class="kw4">void</span> f2<span class="br0">(</span><span class="kw4">int</span> <span class="sy2">*</span>pi, <span class="kw1">struct</span> S <span class="sy2">*</span>ps, <span class="kw1">struct</span> S s<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="co1">// 从 *pi 的读取必须在每次通过 *ps 写入后进行</span>
|
|
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> <span class="sy2">*</span>pi<span class="sy4">;</span> i<span class="sy2">++</span><span class="br0">)</span> <span class="sy2">*</span>ps<span class="sy2">++</span> <span class="sy1">=</span> s<span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>注意 <a href="c-language-restrict.html">restrict 限定符</a>可用于指示二个指针不用作别名使用，即使规则允许它们如此。</p>
|
|
<p>注意类型双关也可以通过<a href="c-language-union.html">联合体</a>的非活跃成员进行。</p>
|
|
<h3><span class="mw-headline" id=".E5.AF.B9.E9.BD.90">对齐</span></h3>
|
|
<p>每个完整<a href="c-language-types.html#.E7.B1.BB.E5.9E.8B.E7.BB.84.E5.88.AB">对象类型</a>拥有一个称作<i>对齐要求</i>的属性，它是一个 <span class="t-lc"><a href="c-types-size_t.html">size_t</a></span> 类型的整数值，表示此类型对象可以分配的相继地址之间的字节数。合法的对齐值是二的非负数次幂。</p>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-c11">
|
|
<td>
|
|
<p>类型的对齐要求可以通过 <a href="c-language-_Alignof.html"><code>_Alignof</code></a> 获得。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<p>为了满足结构体所有对象的对齐要求，一些成员后面可能会插入填充位。</p>
|
|
<div class="t-example">
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="c source-c">
|
|
<pre class="de1"><span class="co2">#include <stdio.h></span>
|
|
<span class="co2">#include <stdalign.h></span>
|
|
|
|
<span class="co1">// struct S 的对象可以分配于任何地址</span>
|
|
<span class="co1">// 因为 S.a 和 S.b 可以分配于任何地址</span>
|
|
<span class="kw1">struct</span> S
|
|
<span class="br0">{</span>
|
|
<span class="kw4">char</span> a<span class="sy4">;</span> <span class="co1">// 成员对象大小： 1 ，对齐： 1</span>
|
|
<span class="kw4">char</span> b<span class="sy4">;</span> <span class="co1">// 成员对象大小： 1 ，对齐： 1</span>
|
|
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 结构体对象大小： 2 ，对齐： 1</span>
|
|
|
|
<span class="co1">// struct X 的对象必须分配于 4字节边界</span>
|
|
<span class="co1">// 因为 X.n 必须分配于 4 字节边界</span>
|
|
<span class="co1">// 因为 int 的对齐要求（通常）是 4</span>
|
|
<span class="kw1">struct</span> X
|
|
<span class="br0">{</span>
|
|
<span class="kw4">int</span> n<span class="sy4">;</span> <span class="co1">// 成员对象大小： 4 ，对齐： 4</span>
|
|
<span class="kw4">char</span> c<span class="sy4">;</span> <span class="co1">// 成员对象大小： 1 ，对齐： 1</span>
|
|
<span class="co1">// 剩余的三个字节进行空位填充</span>
|
|
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 结构体对象大小： 8 ，对齐： 4</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"sizeof(struct S) = %zu<span class="es1">\n</span>"</span>, <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw1">struct</span> S<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"alignof(struct S) = %zu<span class="es1">\n</span>"</span>, alignof<span class="br0">(</span><span class="kw1">struct</span> S<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"sizeof(struct X) = %zu<span class="es1">\n</span>"</span>, <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw1">struct</span> X<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"alignof(struct X) = %zu<span class="es1">\n</span>"</span>, alignof<span class="br0">(</span><span class="kw1">struct</span> X<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>可能的输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">sizeof(struct S) = 2
|
|
alignof(struct S) = 1
|
|
sizeof(struct X) = 8
|
|
alignof(struct X) = 4</pre></div>
|
|
</div>
|
|
</div>
|
|
<p>每个对象类型将其对齐要求强加于该类型的任何一个对象。所有类型中，最严格（最大）的基础对齐是 <span class="t-lc"><a href="c-types-max_align_t.html">max_align_t</a></span> 的对齐。最弱（最小）的对齐是字符类型（ <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">char</span></span></span> 、<span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">signed</span> <span class="kw4">char</span></span></span> 及 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ）的对齐，且等于 1。</p>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-c11">
|
|
<td>
|
|
<p>若用 <a href="c-language-_Alignas.html"><code>_Alignas</code></a> 令一个对象的对齐严格于（大于） <span class="t-lc"><a href="c-types-max_align_t.html">max_align_t</a></span> ，则它拥有<i>扩展对齐要求</i>。成员拥有扩展对齐的结构体或联合体是<i>过对齐类型</i>。是否支持过对齐类型是实现定义的，而且对于每种<a href="c-language-storage_duration.html">存储期</a>的支持可以不同。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-c11">(C11 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
|
|
<div class="t-ref-std-11">
|
|
<ul>
|
|
<li>C11 标准（ISO/IEC 9899:2011）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>3.15 object (p: 6)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.2.6 Representations of types (p: 44-46)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.5/6-7 Expressions (p: 77)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.2.8 Alignment of objects (p: 48-49)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c99">
|
|
<ul>
|
|
<li>C99 标准（ISO/IEC 9899:1999）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>3.2 alignment (p: 3)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>3.14 object (p: 5)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.2.6 Representations of types (p: 37-39)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>6.5/6-7 Expressions (p: 67-68)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c89">
|
|
<ul>
|
|
<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>1.6 Definitions of terms</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin"></table>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|