217 lines
24 KiB
HTML
217 lines
24 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>复制初始化</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_copy_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">复制初始化</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>从另一个对象初始化对象。</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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<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>
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<tr class="t-sdsc">
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<td><span class="t-spar">目标类型</span> <span class="t-spar">对象</span> <code><b>=</b></code> <span class="t-spar">其他对象</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>
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<tr class="t-sdsc">
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<td><span class="t-spar">目标类型</span> <span class="t-spar">对象</span> <code><b>=</b></code> <code><b>{</b></code><span class="t-spar">其他对象</span><code><b>}</b></code> <code><b>;</b></code></td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</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">函数</span><code><b>(</b></code><span class="t-spar">其他对象</span><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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</tr>
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<tr class="t-sdsc">
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<td><code><b>return</b></code> <span class="t-spar">其他对象</span><code><b>;</b></code></td>
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<td>(4)</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>
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<tr class="t-sdsc">
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<td><code><b>throw</b></code> <span class="t-spar">对象</span><code><b>;</b></code>
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<p><code><b>catch (</b></code><span class="t-spar">目标类型</span> <span class="t-spar">对象</span><code><b>)</b></code></p>
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</td>
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<td>(5)</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>
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<tr class="t-sdsc">
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<td><span class="t-spar">目标类型</span> <span class="t-spar">数组</span><code><b>[</b></code><span class="t-spar">元素个数</span><code><b>] = {</b></code><span class="t-spar">其他对象序列</span><code><b>};</b></code></td>
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<td>(6)</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>
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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>复制初始化在下列情况进行：</p>
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<div class="t-li1"><span class="t-li">1)</span> 当声明非引用类型 <span class="t-spar">目标类型</span> 的具名变量（自动、静态或线程局部），带有以等号后随一个表达式所构成的初始化器时。</div>
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<div class="t-li1"><span class="t-li">2)</span> <span class="t-mark-rev t-until-cxx11">(C++11 前)</span>当声明标量类型 <span class="t-spar">目标类型</span> 的具名变量，带有以等号后随一个花括号环绕的表达式所构成的初始化器时（注意：从 C++11 开始，这被归类为<a href="cpp-language-list_initialization.html">列表初始化</a>，且不允许窄化转换）。</div>
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<div class="t-li1"><span class="t-li">3)</span> 当按值<a href="cpp-language-operator_other.html#.E5.86.85.E5.BB.BA.E7.9A.84.E5.87.BD.E6.95.B0.E8.B0.83.E7.94.A8.E8.BF.90.E7.AE.97.E7.AC.A6">传递参数</a>到函数时</div>
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<div class="t-li1"><span class="t-li">4)</span> 当从具有返回值的函数<a href="cpp-language-return.html">返回</a>时</div>
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<div class="t-li1"><span class="t-li">5)</span> 当按值<a href="cpp-language-throw.html">抛出</a>或<a href="cpp-language-try_catch.html">捕获</a>异常时</div>
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<div class="t-li1"><span class="t-li">6)</span> 作为<a href="cpp-language-aggregate_initialization.html">聚合初始化</a>的一部分，以初始化提供了初始化器的每个元素</div>
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<p>复制初始化的效果是：</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx17">
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<td>
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<ul>
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<li>首先，如果 <span class="t-spar">目标类型</span> 是类类型，且初始化器是<a href="cpp-language-value_category.html">纯右值</a>表达式，而其无 cv 限定的类型是与 <span class="t-spar">目标类型</span> 相同的类，那么以初始化器表达式自身而非从它所实质化的临时量初始化目标对象：见<a href="cpp-language-copy_elision.html">复制消除</a></li>
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</ul>
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</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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<ul>
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<li>如果 <span class="t-spar">目标类型</span> 是类类型，且 <span class="t-spar">其他对象</span> 的类型的无 cv 限定版本是 <span class="t-spar">目标类型</span> 或是从 <span class="t-spar">目标类型</span> 派生的类，那么检测 <span class="t-spar">目标类型</span> 的<a href="cpp-language-converting_constructor.html">非显式构造函数</a>，并由重载决议选择最佳匹配。然后调用构造函数以初始化该对象。</li>
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</ul>
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<ul>
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<li>如果 <span class="t-spar">目标类型</span> 是类类型，且 <span class="t-spar">其他对象</span> 的类型的无 cv 限定版本不是 <span class="t-spar">目标类型</span> 或从 <span class="t-spar">目标类型</span> 派生，或如果 <span class="t-spar">目标类型</span> 是非类类型，但 <span class="t-spar">其他对象</span> 的类型是类类型，那么检测能从 <span class="t-spar">其他对象</span> 的类型转换到 <span class="t-spar">目标类型</span> （或当 <span class="t-spar">目标类型</span> 是类类型且存在转换函数时，转换到从 <span class="t-spar">目标类型</span> 派生的类型）的<a href="cpp-language-implicit_conversion.html">用户定义转换序列</a>，并通过重载决议选择最佳者。该转换的结果（如果使用<a href="cpp-language-converting_constructor.html">转换构造函数</a>，那么就是无cv 限定的 <span class="t-spar">目标类型</span> 的<span class="t-rev-inl t-until-cxx17"><span>纯右值临时量</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span><span class="t-rev-inl t-since-cxx17"><span>纯右值表达式</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>）会被用于<a href="cpp-language-direct_initialization.html">直接初始化</a>该对象。<span class="t-rev-inl t-until-cxx17"><span>最后一步通常会被<a href="cpp-language-copy_elision.html">优化掉</a>，而直接将转换结果构造于分配给目标对象的内存之中，但即便不会使用，也要求适合的构造函数（移动或复制）是可以访问的。</span> <span><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span></li>
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</ul>
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<ul>
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<li>否则（如果 <span class="t-spar">目标类型</span> 和 <span class="t-spar">其他对象</span> 的类型都不是类类型），需要时用<a href="cpp-language-implicit_conversion.html">标准转换</a>将 <span class="t-spar">其他对象</span> 的值转换成 <span class="t-spar">目标类型</span> 的无 cv 限定版本。</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>复制初始化不如直接初始化宽容：<a href="cpp-language-explicit.html">explicit 构造函数</a>不是<a href="cpp-language-converting_constructor.html">转换构造函数</a>，也不会被复制初始化考虑。</p>
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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="kw1">struct</span> Exp <span class="br0">{</span> <span class="kw1">explicit</span> Exp<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 不能从 const char* 转换</span>
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Exp e1<span class="br0">(</span><span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
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Exp e2 <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span> <span class="co1">// 错误，复制初始化不考虑 explicit 构造函数</span>
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<span class="kw1">struct</span> Imp <span class="br0">{</span> Imp<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 可以从 const char* 转换</span>
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Imp i1<span class="br0">(</span><span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span>
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Imp i2 <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span> <span class="co1">// OK</span></pre></div>
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</div>
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<p>另外，复制初始化中的隐式转换必须从初始化器直接生成 <span class="t-spar">目标类型</span>，然而直接初始化会期待一个从初始化器到 <span class="t-spar">目标类型</span> 的构造函数实参的隐式转换。</p>
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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="kw1">struct</span> S <span class="br0">{</span> S<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 可以从 std::string 转换</span>
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S s<span class="br0">(</span><span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：从 const char[4] 转换到 std::string</span>
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S s <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span> <span class="co1">// 错误：没有从 const char[4] 到 S 的转换</span>
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S s <span class="sy1">=</span> <span class="st0">"abc"</span>s<span class="sy4">;</span> <span class="co1">// OK：从 std::string 转换到 S</span></pre></div>
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</div>
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<p>如果 <span class="t-spar">其他对象</span> 是右值表达式，那么重载决议会选择<a href="cpp-language-move_constructor.html">移动构造函数</a>并在复制初始化期间调用它。没有“移动初始化”的概念。</p>
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<p><a href="cpp-language-implicit_conversion.html">隐式转换</a>以复制初始化的方式定义：如果能以表达式 <code>E</code> 复制初始化 <span class="t-spar">目标类型</span> 类型对象，那么可以隐式转换 <code>E</code> 到 <span class="t-spar">目标类型</span>。</p>
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<p>具名变量的复制初始化中的等号 <code><b>=</b></code> 与赋值运算符无关。重载的赋值运算符对复制初始化没有影响。</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 <utility></span>
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<span class="co2">#include <memory></span>
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<span class="kw1">struct</span> A
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<span class="br0">{</span>
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operator <span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">12</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> B
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<span class="br0">{</span>
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B<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</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="sy1">=</span> <span class="st0">"test"</span><span class="sy4">;</span> <span class="co1">// OK：构造函数非显式</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 此复制初始化进行移动</span>
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<span class="co1">// std::unique_ptr<int> p = new int(1); // 错误：构造函数是显式的</span>
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<a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> p<span class="br0">(</span>new <span class="kw4">int</span><span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：直接初始化</span>
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<span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu16">3.14</span><span class="sy4">;</span> <span class="co1">// 浮点整型转换</span>
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<span class="kw4">const</span> <span class="kw4">int</span> b <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// 不受 const 影响</span>
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<span class="kw4">int</span> c <span class="sy1">=</span> b<span class="sy4">;</span> <span class="co1">// ……反之亦然</span>
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A a<span class="sy4">;</span>
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B b0 <span class="sy1">=</span> <span class="nu0">12</span><span class="sy4">;</span>
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<span class="co1">// B b1 = a; // < 错误：要求从 'A' 到非标量类型 'B' 的转换</span>
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B b2<span class="br0">{</span>a<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// < 等同：调用 A::operator int() ，然后是::B(int)</span>
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B b3 <span class="sy1">=</span> <span class="br0">{</span>a<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// <</span>
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<span class="kw4">auto</span> b4 <span class="sy1">=</span> B<span class="br0">{</span>a<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// <</span>
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<span class="co1">// b0 = a; // < 错误：需要重载赋值运算符</span>
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<span class="br0">}</span></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/issue5">CWG 5</a></td>
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<td>C++98</td>
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<td>目标类型的 cv 限定会应用到转换构造函数初始化的临时量</td>
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<td>临时量不再有 cv 限定</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-copy_elision.html">复制消除</a></li>
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<li><a href="cpp-language-converting_constructor.html">转换构造函数</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-copy_constructor.html">复制构造函数</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-destructor.html">析构函数</a></li>
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<li><a href="cpp-language-explicit.html">explicit</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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