127 lines
10 KiB
HTML
127 lines
10 KiB
HTML
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<title>std::memcmp</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_string_byte_memcmp 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>memcmp</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-string.html">string</a>‎ | <a href="cpp-string-byte.html">byte</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-cstring.html"><cstring></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> memcmp<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">void</span><span class="sy2">*</span> lhs, <span class="kw4">const</span> <span class="kw4">void</span><span class="sy2">*</span> rhs, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> count <span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>转译 <code>lhs</code> 和 <code>rhs</code> 所指向的对象为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 数组，并比较这些数组的首 <code>count</code> 个字符。按字典序比较。</p>
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<p>结果的符号是在被比较对象中相异的首对字节的值（都转译成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ）的差。</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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<table class="t-par-begin">
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<td>lhs, rhs</td>
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<td>-</td>
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<td>指向要比较的内存缓冲区的指针</td>
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</tr>
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<tr class="t-par">
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<td>count</td>
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<td>-</td>
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<td>要检验的字节数</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>若 <code>lhs</code> 中首个相异字节（转译为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ）小于 <code>rhs</code> 中的对应字节则为负值。</p>
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<p>若 <code>lhs</code> 和 <code>rhs</code> 的所有 <code>count</code> 个字节相等则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 。</p>
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<p>若 <code>lhs</code> 中首个相异字节大于 <code>rhs</code> 中的对应字节则为正值。</p>
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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>此函数读取<a href="c-language-object.html">对象表示</a>，而非对象值，而且典型地只对无填充的可平凡复制对象有意义。例如二个 <span class="t-lc"><a href="cpp-string-basic_string.html">std::string</a></span> 或 <span class="t-lc"><a href="cpp-container-vector.html">std::vector</a></span> 类型对象间的 <code>memcmp()</code> 将不比较其内容，而二个 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span><span class="br0">{</span><span class="kw4">char</span> c<span class="sy4">;</span> <span class="kw4">int</span> n<span class="sy4">;</span></span></span>} 类型对象间的 <code>memcmp()</code> 将比较填充字节，其值在 <span class="t-c"><span class="mw-geshi cpp source-cpp">c</span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">n</span></span> 相同时可以相异。</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 <cstring></span>
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<span class="kw4">void</span> demo<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> lhs, <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> rhs, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> sz<span class="br0">)</span>
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<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> <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="br0">(</span>lhs, sz<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> rc <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">memcmp</span><span class="br0">(</span>lhs, rhs, sz<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>rc <span class="sy1">==</span> <span class="nu0">0</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">" compares equal to "</span><span class="sy4">;</span>
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<span class="kw1">else</span> <span class="kw1">if</span><span class="br0">(</span>rc <span class="sy1"><</span> <span class="nu0">0</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">" precedes "</span><span class="sy4">;</span>
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<span class="kw1">else</span> <span class="kw1">if</span><span class="br0">(</span>rc <span class="sy1">></span> <span class="nu0">0</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">" follows "</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> <a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="br0">(</span>rhs, sz<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" in lexicographical order<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</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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<span class="kw4">char</span> a1<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'a'</span>,<span class="st0">'b'</span>,<span class="st0">'c'</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">char</span> a2<span class="br0">[</span>sizeof a1<span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'a'</span>,<span class="st0">'b'</span>,<span class="st0">'d'</span><span class="br0">}</span><span class="sy4">;</span>
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demo<span class="br0">(</span>a1, a2, sizeof a1<span class="br0">)</span><span class="sy4">;</span>
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demo<span class="br0">(</span>a2, a1, sizeof a1<span class="br0">)</span><span class="sy4">;</span>
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demo<span class="br0">(</span>a1, a1, sizeof a1<span class="br0">)</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">abc precedes abd in lexicographical order
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abd follows abc in lexicographical order
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abc compares equal to abc in lexicographical order</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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-byte-strcmp.html"><span class="t-lines"><span>strcmp</span></span></a></div>
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</div>
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</td>
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<td>比较两个字符串<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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<div class="t-dsc-member-div">
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<div><a href="cpp-string-byte-strncmp.html"><span class="t-lines"><span>strncmp</span></span></a></div>
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</div>
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</td>
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<td>比较两个字符串的一定量字符<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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</div>
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<div class="visualClear"></div>
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