152 lines
13 KiB
HTML
152 lines
13 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>零初始化</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_zero_initialization skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading">零初始化</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-language.html">language</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<p>将一个对象的初始值设为零。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<p>注意零初始化在语言中没有专用语法，因此下列语法不是零初始化语法。这些是可能会进行零初始化的其他初始化的例子。</p>
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<table class="t-sdsc-begin">
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><code><b>static</b></code> <span class="t-spar">T</span> <span class="t-spar">对象</span> <code><b>;</b></code></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><span class="t-spar">T</span> <code><b>()</b></code> <code><b>;</b></code>
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<p><span class="t-spar">T</span> <span class="t-spar">t</span> <code><b>=</b></code> <code><b>{}</b></code> <code><b>;</b></code><br />
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<span class="t-spar">T</span> <code><b>{}</b></code> <code><b>;</b></code></p>
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</td>
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<td>(2)</td>
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<td><br />
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<br />
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<span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><span class="t-spar">CharT</span> <span class="t-spar">数组</span> <code><b>[</b></code> <span class="t-spar">n</span> <code><b>]</b></code> <code><b>= "";</b></code></td>
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<td>(3)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</table>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p>在下列情形进行零初始化：</p>
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<div class="t-li1"><span class="t-li">1)</span> 在所有其他初始化前，对每个具有静态<span class="t-rev-inl t-since-cxx11"><span>或线程局部</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span><a href="cpp-language-storage_duration.html">存储期</a>的，不进行<a href="cpp-language-constant_initialization.html">常量初始化</a>的具名变量。</div>
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<div class="t-li1"><span class="t-li">2)</span> 作为非类类型变量，和被值初始化的无构造函数的类类型的成员的<a href="cpp-language-value_initialization.html">值初始化</a>序列的一部分，包括未提供初始化器的<a href="cpp-language-aggregate_initialization.html">聚合体</a>元素的值初始化。</div>
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<div class="t-li1"><span class="t-li">3)</span> 以不够长的<a href="cpp-language-aggregate_initialization.html#.E5.AD.97.E7.AC.A6.E6.95.B0.E7.BB.84">字符串字面量初始化</a>任何<a href="cpp-language-types.html#.E5.AD.97.E7.AC.A6.E7.B1.BB.E5.9E.8B">字符类型</a>数组时，零初始化数组的剩余部分。</div>
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<p>零初始化的效果是：</p>
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<ul>
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<li>如果 <code>T</code> 是标量类型，那么对象初值是<a href="cpp-language-explicit_cast.html">显式转换</a>到 <code>T</code> 的整型常量零。</li>
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</ul>
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<ul>
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<li>如果 <code>T</code> 是非联合体类类型，那么零初始化它的所有基类和非静态数据成员，并初始化所有填充位为零位。忽略可能存在的构造函数。</li>
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</ul>
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<ul>
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<li>如果 <code>T</code> 是联合体类型，那么零初始化首个非静态的具名数据成员，并零初始化所有填充位为零位。</li>
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</ul>
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<ul>
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<li>如果 <code>T</code> 是数组类型，那么零初始化每个元素。</li>
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</ul>
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<ul>
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<li>如果 <code>T</code> 是引用类型，那么不做任何事。</li>
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</ul>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>如<a href="cpp-language-initialization.html#.E9.9D.9E.E5.B1.80.E9.83.A8.E5.8F.98.E9.87.8F">非局部初始化</a>中所述，在进行任何其他初始化前，零初始化所有未被常量初始化的静态<span class="t-rev-inl t-since-cxx11"><span>和线程局部</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>变量。如果非类类型非局部变量的定义没有初始化器，那么默认初始化不做任何事，不修改先前零初始化的结果。</p>
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<p>零初始化的指针是其类型的空指针值，即使空指针的值并非整型的零。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <string></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> A <span class="br0">{</span>
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<span class="kw4">int</span> a,b,c<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">double</span> f<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 零初始化为三个 0.0</span>
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<span class="kw4">int</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// 零初始化为空指针值（即使该值可能不是整型 0）</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span> <span class="co1">// 零初始化为不确定值</span>
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<span class="co1">// 再通过 std::string 的默认构造函数默认初始化为 ""</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">int</span> argc, <span class="kw4">char</span><span class="sy2">*</span> argv<span class="br0">[</span><span class="br0">]</span><span class="br0">)</span>
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<span class="br0">{</span>
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delete p<span class="sy4">;</span> <span class="co1">// 可以安全删除空指针</span>
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<span class="kw4">static</span> <span class="kw4">int</span> n <span class="sy1">=</span> argc<span class="sy4">;</span> <span class="co1">// 零初始化为 0</span>
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<span class="co1">// 然后复制初始化为 argc</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"n = "</span> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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A a <span class="sy1">=</span> A<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 效果等同于 A a{}; 或 A a = {};</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"a = {"</span> <span class="sy1"><<</span> a.<span class="me1">a</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> a.<span class="me1">b</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> a.<span class="me1">c</span> <span class="sy1"><<</span> <span class="st0">"}<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">n = 1
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a = {0 0 0}</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue2026">CWG 2026</a></td>
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<td>C++98</td>
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<td>曾指定始终首先进行零初始化，甚至先于常量初始化</td>
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<td>如果适用常量初始化则不进行零初始化</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<ul>
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<li><a href="cpp-language-constructor.html">构造函数</a></li>
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<li><a href="cpp-language-copy_assignment.html">复制赋值</a></li>
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<li><a href="cpp-language-default_constructor.html">默认构造函数</a></li>
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<li><a href="cpp-language-initialization.html">初始化</a>
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<ul>
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<li><a href="cpp-language-aggregate_initialization.html">聚合初始化</a></li>
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<li><a href="cpp-language-constant_initialization.html">常量初始化</a></li>
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<li><a href="cpp-language-copy_initialization.html">复制初始化</a></li>
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<li><a href="cpp-language-default_initialization.html">默认初始化</a></li>
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<li><a href="cpp-language-direct_initialization.html">直接初始化</a></li>
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<li><a href="cpp-language-list_initialization.html">列表初始化</a></li>
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<li><a href="cpp-language-value_initialization.html">值初始化</a></li>
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</ul>
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</li>
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<li><a href="cpp-language-move_assignment.html">移动赋值</a></li>
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<li><a href="cpp-language-new.html">new</a></li>
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</ul>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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