134 lines
13 KiB
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134 lines
13 KiB
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<title>assert</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_error_assert skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">assert</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-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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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-cassert.html"><cassert></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="co2">#ifdef NDEBUG</span><br /></span>
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<p><span class="co2">#define assert(condition) ((void)0)</span><br />
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<span class="co2">#else</span><br />
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<span class="co2">#define assert(condition) /*implementation defined*/</span><br /></p>
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<span class="co2">#endif</span></div>
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<p>宏 <code>assert</code> 的定义依赖于标准库中没有定义的另一个名为 <span class="t-c"><span class="mw-geshi cpp source-cpp">NDEBUG</span></span> 宏。</p>
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<p>若在包含了 <a href="cpp-header-cassert.html"><tt><cassert></tt></a> 或 <code><assert.h></code> 的源代码中定义名为 <span class="t-c"><span class="mw-geshi cpp source-cpp">NDEBUG</span></span> 的宏，则 <code>assert</code> 不做任何事。</p>
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<p>若没有定义名为 <span class="t-c"><span class="mw-geshi cpp source-cpp">NDEBUG</span></span> 的宏，则 <code>assert</code> 将其参数（必须拥有标量类型）与零比较相等。若相等，则 <code>assert</code> 在标准错误输出上输出实现指定的诊断信息，并调用 <span class="t-lc"><a href="cpp-utility-program-abort.html">std::abort</a></span> 。诊断信息要求包含 <code>expression</code> 的文本，还有<span class="t-rev-inl t-since-cxx11"><span><a href="cpp-language-function.html">预定义变量</a> <span class="t-c"><span class="mw-geshi cpp source-cpp">__func__</span></span> 与</span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span><a href="cpp-preprocessor-replace.html">预定义宏</a> <span class="t-c"><span class="mw-geshi cpp source-cpp">__FILE__</span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">__LINE__</span></span> 的值。</p>
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<table class="t-rev-begin">
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">assert<span class="br0">(</span>E<span class="br0">)</span></span></span> 保证为<a href="cpp-language-constant_expression.html">常量子表达式</a>，若下列之一成立</p>
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<ul>
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<li>在最后定义或重定义（即包含了头文件 <code><cassert></code> 或 <code><assert.h></code> ） <code>assert</code> 位置定义了 <span class="t-c"><span class="mw-geshi cpp source-cpp">NDEBUG</span></span>；或</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> 按语境转换成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 后，是求值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 的常量子表达式。</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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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>condition</td>
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<td>-</td>
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<td>标量类型的表达式</td>
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</tr>
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</table>
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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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<p>（无）</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>因为 <code>assert</code> 是<a href="cpp-preprocessor-replace.html">仿函数宏</a>，在 <span class="t-spar">condition</span> 中未被括号保护的逗号都被转译成宏参数的分隔符。这种逗号常能在模板参数列表和列表初始化中找到：</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">assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误： assert 不接收二个参数</span>
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assert<span class="br0">(</span><span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">></span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK ：一个参数</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">></span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK ：非宏</span>
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<a href="cpp-numeric-complex.html"><span class="kw1471">std::<span class="me2">complex</span></span></a><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span> c<span class="sy4">;</span>
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assert<span class="br0">(</span>c <span class="sy1">==</span> <a href="cpp-numeric-complex.html"><span class="kw1471">std::<span class="me2">complex</span></span></a><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span><span class="br0">{</span><span class="nu0">0</span>, <span class="nu0">0</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误</span>
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assert<span class="br0">(</span><span class="br0">(</span>c <span class="sy1">==</span> <a href="cpp-numeric-complex.html"><span class="kw1471">std::<span class="me2">complex</span></span></a><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span><span class="br0">{</span><span class="nu0">0</span>, <span class="nu0">0</span><span class="br0">}</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</span></pre></div>
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<p>没有标准化的添加消息到 <code>assert</code> 错误的接口。一个包含它的方式是使用<a href="cpp-language-operator_other.html#.E5.86.85.E5.BB.BA.E7.9A.84.E9.80.97.E5.8F.B7.E8.BF.90.E7.AE.97.E7.AC.A6">逗号运算符</a>，只要它未被<a href="cpp-language-operators.html">重载</a>：</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">assert<span class="br0">(</span><span class="br0">(</span><span class="st0">"There are five lights"</span>, <span class="nu0">2</span> <span class="sy2">+</span> <span class="nu0">2</span> <span class="sy1">==</span> <span class="nu0">5</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></pre></div>
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<p><code>assert</code> 在 <a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/assert-macro-assert-wassert?view=msvc-160">Microsoft CRT</a> 中的实现不遵从 C++11 以及后续标准版本，因为其底层函数 ( <code>_wassert</code> ）不接收 <span class="t-c"><span class="mw-geshi cpp source-cpp">__func__</span></span> 或等价的替代品。</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="co1">// 去注释化则禁用 assert()</span>
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<span class="co1">// #define NDEBUG</span>
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<span class="co2">#include <cassert></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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assert<span class="br0">(</span><span class="nu0">2</span><span class="sy2">+</span><span class="nu0">2</span><span class="sy1">==</span><span class="nu0">4</span><span class="br0">)</span><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">"Execution continues past the first assert<span class="es1">\n</span>"</span><span class="sy4">;</span>
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assert<span class="br0">(</span><span class="nu0">2</span><span class="sy2">+</span><span class="nu0">2</span><span class="sy1">==</span><span class="nu0">5</span><span class="br0">)</span><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">"Execution continues past the second assert<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>可能的输出：</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">Execution continues past the first assert
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test: test.cc:10: int main(): Assertion `2+2==5' failed.
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Aborted</pre></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><a href="cpp-language-static_assert.html"><code>static_assert</code> 声明</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>进行编译时断言检查</td>
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</tr>
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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-utility-program-abort.html"><span class="t-lines"><span>abort</span></span></a></div>
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</div>
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</td>
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<td>导致非正常的程序终止（不进行清理）<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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