129 lines
15 KiB
HTML
129 lines
15 KiB
HTML
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<title>noexcept 运算符 (C++11 起)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_noexcept skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">noexcept 运算符 <span class="t-mark-rev t-since-cxx11">(C++11 起)</span></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><code>noexcept</code> 运算符进行编译时检查，如果表达式不会抛出任何异常则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>。</p>
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<p>它可用于函数模板的 <a href="cpp-language-noexcept_spec.html"><code>noexcept</code> 说明符</a>中，以声明函数将对某些类型抛出异常，但不对其他类型抛出。</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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<td class="t-sdsc-nopad"><code><b>noexcept(</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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<p>返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 类型的<a href="cpp-language-value_category.html">纯右值</a>。</p>
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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><code>noexcept</code> 运算符不会对 <span class="t-spar">表达式</span> 求值。</p>
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<p>如果 <span class="t-rev-inl t-until-cxx17"><span><span class="t-spar">表达式</span> 的<a href="cpp-language-except_spec.html"><i>潜在异常</i></a>集合为空</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><span class="t-spar">表达式</span> <a href="cpp-language-noexcept_spec.html"><i>不会抛出</i></a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>，那么结果是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span>，否则结果是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span>。</p>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="cpp-keyword-noexcept.html"><tt>noexcept</tt></a></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 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 <iostream></span>
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<span class="co2">#include <utility></span>
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<span class="co2">#include <vector></span>
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<span class="kw4">void</span> may_throw<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">void</span> no_throw<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">auto</span> lmay_throw <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> lno_throw <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">class</span> T
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<span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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~T<span class="br0">(</span><span class="br0">)</span><span class="br0">{</span><span class="br0">}</span> <span class="co1">// 析构函数妨碍了移动构造函数</span>
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<span class="co1">// 复制构造函数不会抛出异常</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">class</span> U
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<span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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~U<span class="br0">(</span><span class="br0">)</span><span class="br0">{</span><span class="br0">}</span> <span class="co1">// 析构函数妨碍了移动构造函数</span>
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<span class="co1">// 复制构造函数可能会抛出异常</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">class</span> V
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<span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="sy4">;</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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T t<span class="sy4">;</span>
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U u<span class="sy4">;</span>
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V v<span class="sy4">;</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> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a>
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<span class="sy1"><<</span> <span class="st0">"may_throw() 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>may_throw<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"no_throw() 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>no_throw<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"lmay_throw() 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>lmay_throw<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"lno_throw() 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>lno_throw<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"~T() 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>.~T<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="co1">// 注：以下各项测试也要求 ~T() 不会抛出异常</span>
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<span class="co1">// 因为 noexcept 中的表达式会构造并销毁临时量</span>
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<span class="sy1"><<</span> <span class="st0">"T(T 右值) 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>T<span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"T(T 左值) 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>T<span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"U(U 右值) 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>U<span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"U(U 左值) 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>U<span class="br0">(</span>u<span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"V(V 右值) 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>V<span class="br0">(</span><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>V<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"V(V 左值) 可能会抛出异常吗？"</span> <span class="sy1"><<</span> <span class="sy3">!</span><span class="kw1">noexcept</span><span class="br0">(</span>V<span class="br0">(</span>v<span class="br0">)</span><span class="br0">)</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">may_throw() 可能会抛出异常吗？true
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no_throw() 可能会抛出异常吗？false
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lmay_throw() 可能会抛出异常吗？true
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lno_throw() 可能会抛出异常吗？false
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~T() 可能会抛出异常吗？false
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T(T 右值) 可能会抛出异常吗？false
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T(T 左值) 可能会抛出异常吗？false
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U(U 右值) 可能会抛出异常吗？true
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U(U 左值) 可能会抛出异常吗？true
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V(V 右值) 可能会抛出异常吗？false
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V(V 左值) 可能会抛出异常吗？true</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td><a href="cpp-language-noexcept_spec.html"><code>noexcept</code> 说明符</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>指定函数是否抛出异常</td>
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</tr>
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<tr class="t-dsc">
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<td><a href="cpp-language-except_spec.html">动态异常说明</a><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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<td>指定函数所抛出的异常 <span class="t-mark-rev t-deprecated-cxx11">(C++11 中弃用)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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