156 lines
12 KiB
HTML
156 lines
12 KiB
HTML
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<title>std::uncaught_exception, std::uncaught_exceptions</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_error_uncaught_exception skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>uncaught_exception, <span style="font-size:0.7em; line-height:130%">std::</span>uncaught_exceptions</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-error.html">error</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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<div>定义于头文件 <code><a href="cpp-header-exception.html"><exception></a></code></div>
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<td></td>
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<td></td>
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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> uncaught_exception<span class="br0">(</span><span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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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 class="t-dcl t-since-cxx11 t-until-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> uncaught_exception<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
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<span class="t-mark">(C++17 中弃用)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 中移除)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> uncaught_exceptions<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 检测当前线程是否有生存的异常对象，即被抛出或重抛出且未进入匹配的 catch 子句、 <span class="t-lc"><a href="cpp-error-terminate.html">std::terminate</a></span> 或 <span class="t-lc"><a href="cpp-error-unexpected.html">std::unexpected</a></span> 的异常。换言之，<code>std::uncaught_exception</code> 检测当前是否在进行<a href="cpp-language-throw.html#.E6.A0.88.E5.9B.9E.E6.BA.AF">栈回溯</a>。</div>
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<div class="t-li1"><span class="t-li">2)</span> 检测当前线程已经抛出或重抛出且未进入其匹配 catch 子句的异常对象数。</div>
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<p>有时抛出异常是安全的，即使当 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">uncaught_exception</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">true</span></span></span> 。例如，若<a href="cpp-language-throw.html#.E6.A0.88.E5.9B.9E.E6.BA.AF">栈回溯</a>导致要析构对象，则该对象的析构函数可以运行抛出异常的代码，只要在离开析构函数前为某 catch 块捕捉该异常。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<p>（无）</p>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> 若此线程中当前正在进行栈回溯则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 当前线程中的为捕捉异常对象数。</div>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>返回 int 的 <code>uncaught_exceptions</code> 的一个使用例子是 <a rel="nofollow" class="external text" href="http://www.boost.org/doc/libs/release/libs/log/doc/html/index.html">boost.log</a> 库：表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">BOOST_LOG<span class="br0">(</span>logger<span class="br0">)</span> <span class="sy1"><<</span> foo<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></span> 首先创建保障对象并记录其构造函数中的未捕捉异常数。由保障对象的析构函数进行输出，除非 foo() 抛出（该情况下析构函数中未捕捉异常的数量大于构造函数所观察到的）</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 <iostream></span>
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<span class="co2">#include <exception></span>
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<span class="co2">#include <stdexcept></span>
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<span class="kw1">struct</span> Foo <span class="br0">{</span>
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<span class="kw4">int</span> count <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">uncaught_exceptions</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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~Foo<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="br0">(</span>count <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">uncaught_exceptions</span><span class="br0">(</span><span class="br0">)</span>
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<span class="sy4">?</span> <span class="st0">"~Foo() called normally<span class="es1">\n</span>"</span>
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<span class="sy4">:</span> <span class="st0">"~Foo() called during stack unwinding<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<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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Foo f<span class="sy4">;</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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Foo f<span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Exception thrown<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">throw</span> <a href="cpp-error-runtime_error.html"><span class="kw770">std::<span class="me2">runtime_error</span></span></a><span class="br0">(</span><span class="st0">"test exception"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</span><span class="kw4">const</span> <a href="cpp-error-exception.html"><span class="kw764">std::<span class="me2">exception</span></span></a><span class="sy3">&</span> e<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Exception caught: "</span> <span class="sy1"><<</span> e.<span class="me1">what</span><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>
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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">Exception thrown
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~Foo() called during stack unwinding
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Exception caught: test exception
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~Foo() called normally</pre></div>
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</div>
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</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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-error-terminate.html"><span class="t-lines"><span>terminate</span></span></a></div>
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</div>
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</td>
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<td>异常处理失败时调用的函数<br />
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<span class="t-mark">(函数)</span></td>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-error-exception_ptr.html"><span class="t-lines"><span>exception_ptr</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>用于处理异常对象的共享指针类型<br />
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<span class="t-mark">(typedef)</span></td>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-error-current_exception.html"><span class="t-lines"><span>current_exception</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>捕获当前异常到 <span class="t-lc"><a href="cpp-error-exception_ptr.html">std::exception_ptr</a></span> 之中<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.A4.96.E9.83.A8.E9.93.BE.E6.8E.A5">外部链接</span></h3>
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<ul>
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<li><a rel="nofollow" class="external text" href="http://www.gotw.ca/gotw/047.htm">GOTW 问题 47 ：未捕捉异常</a></li>
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<li><a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4152.pdf"><span class="t-lc">std::uncaught_exceptions</span> 的基本原理</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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</body>
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