119 lines
16 KiB
HTML
119 lines
16 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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<title>函数 try 块</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_function-try-block 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">函数 try 块</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>建立围绕整个函数体的异常处理块。</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>函数 try 块是一种 <span class="t-spar">函数体</span> 的替代语法形式，它是<a href="cpp-language-function.html">函数定义</a>的一部分。</p>
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<td class="t-sdsc-nopad"><code><b>try</b></code> <span class="t-spar">构造函数初始化器</span><span class="t-mark">(可选)</span> <span class="t-spar">复合语句</span> <span class="t-spar">处理块序列</span></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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<td colspan="10" class="t-sdsc-sep"></td>
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</table>
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<table class="t-par-begin">
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<tr class="t-par">
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<td><span class="t-spar">构造函数初始化器</span></td>
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<td>-</td>
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<td>成员<a href="cpp-language-initializer_list.html" class="mw-redirect">初始化器列表</a>，只能用于构造函数</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">复合语句</span></td>
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<td>-</td>
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<td>花括号环绕的<a href="cpp-language-statements.html#.E5.A4.8D.E5.90.88.E8.AF.AD.E5.8F.A5">语句序列</a>，它组成函数体</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">处理块序列</span></td>
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<td>-</td>
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<td>一或多个 <a href="cpp-language-try_catch.html">catch 子句</a>的序列</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><i>函数 try 块</i>将一系列 catch 子句与整个函数体，以及<a href="cpp-language-initializer_list.html" class="mw-redirect">成员初始化器列表</a>（如果用于构造函数）关联起来。从函数体中的任何语句，或（对于构造函数）从任何成员或基类的构造函数，或（对于析构函数）从任何成员或基类的析构函数中抛出的所有异常，以与常规 <a href="cpp-language-try_catch.html">try 块</a>中抛出的异常时相同方式，将控制转移到 <span class="t-spar">处理块序列</span> 中。</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="co2">#include <iostream></span>
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<span class="co2">#include <string></span>
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<span class="kw1">struct</span> S <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> m<span class="sy4">;</span>
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S<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy3">&</span> str, <span class="kw4">int</span> idx<span class="br0">)</span> <span class="kw1">try</span> <span class="sy4">:</span> m<span class="br0">(</span>str, idx<span class="br0">)</span> <span class="br0">{</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">"S("</span> <span class="sy1"><<</span> str <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> idx <span class="sy1"><<</span> <span class="st0">") 构造完成，m = "</span> <span class="sy1"><<</span> m <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> <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="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"S("</span> <span class="sy1"><<</span> str <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> idx <span class="sy1"><<</span> <span class="st0">") 失败："</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> <span class="co1">// 此处有隐含的 "throw;"</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> <span class="br0">{</span>
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S s1<span class="br0">{</span><span class="st0">"ABC"</span>, <span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 不抛出（索引在范围内）</span>
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<span class="kw1">try</span> <span class="br0">{</span>
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S s2<span class="br0">{</span><span class="st0">"ABC"</span>, <span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 抛出（越界）</span>
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<span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</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="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"S s2... 抛出了一个异常："</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>在进入任何构造函数上的函数 try 块的 catch 子句前，所有完整构造的成员和基类都会被销毁。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<p>如果函数 try 块在委托构造函数上，而它调用的非委托构造函数成功完成，但之后该委托构造函数的函数体抛出了异常，那么此对象的析构函数将在进入函数 try 块的任何 catch 子句之前完成。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<p>在进入析构函数上的函数 try 块的任何 catch 子句之前，所有基类和非变体成员均已被销毁。</p>
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<p>如果用于构造函数或析构函数上的函数 try 块的 catch 子句访问了基类或对象的非静态成员，那么行为未定义。</p>
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<p>构造函数的函数 try 块中的每个 catch 子句都必须以抛异常终止。如果控制抵达这种异常处理的尾部，那么当前异常被如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">throw</span><span class="sy4">;</span></span></span> 来自动重抛。在构造函数的函数 try 块的任何 catch 子句中都不能使用 return 语句。</p>
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<p>抵达析构函数的函数 try 块的 catch 子句末尾，也如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">throw</span><span class="sy4">;</span></span></span> 来自动重抛当前异常，但可以使用 return 语句。</p>
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<p>对于所有其他函数，如果函数返回类型是（可有 cv 限定的）<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span>，那么抵达 catch 子句末尾等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span><span class="sy4">;</span></span></span>，否则行为未定义。</p>
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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>函数 try 块的主要目的是应对从构造函数中的成员初始化器列表抛出的异常，进行记录并重抛，修改异常对象并重抛，抛出一个不同的异常，或终止程序。析构函数或常规函数很少用到它们。</p>
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<p>函数 try 块不会捕获从按值传递的函数形参的复制/移动构造函数和析构函数中抛出的异常：这些异常是在调用方的语境抛出的。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<p>线程的顶层函数的函数 try 块不会捕获从<a href="cpp-language-storage_duration.html">线程局部</a>对象的构造函数和析构函数（但除了函数作用域的线程局域对象的构造函数）中抛出的异常。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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</table>
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<p>类似地，<a href="cpp-language-main_function.html">main() 函数</a>的函数 try 块不会捕获从<a href="cpp-language-storage_duration.html">静态</a>对象的构造函数和析构函数（但除了函数局域静态对象的构造函数）中抛出的异常。</p>
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<p>函数形参（但并非在函数自身中声明的任何对象）的<a href="cpp-language-scope.html#.E5.9D.97.E4.BD.9C.E7.94.A8.E5.9F.9F">作用域</a>和生存期延续到 <span class="t-spar">处理块序列</span> 的结尾。</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="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">2</span><span class="br0">)</span> <span class="kw1">try</span> <span class="br0">{</span>
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<span class="sy2">++</span>n<span class="sy4">;</span> <span class="co1">// 使函数形参自增</span>
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<span class="kw1">throw</span> n<span class="sy4">;</span>
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<span class="br0">}</span> <span class="kw1">catch</span><span class="br0">(</span>...<span class="br0">)</span> <span class="br0">{</span>
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<span class="sy2">++</span>n<span class="sy4">;</span> <span class="co1">// n 处于作用域中且仍然指代函数形参</span>
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<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>n <span class="sy1">==</span> <span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> n<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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