156 lines
9.9 KiB
HTML
156 lines
9.9 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>std::memmove</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_string_byte_memmove 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"><span style="font-size:0.7em; line-height:130%">std::</span>memmove</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-string.html">string</a>‎ | <a href="cpp-string-byte.html">byte</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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<td>
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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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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span> memmove<span class="br0">(</span> <span class="kw4">void</span><span class="sy2">*</span> dest, <span class="kw4">const</span> <span class="kw4">void</span><span class="sy2">*</span> src, <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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<td class="t-dcl-nopad"></td>
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<td class="t-dcl-nopad"></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>从 <code>src</code> 所指向的对象复制 <code>count</code> 个字节到 <code>dest</code> 所指向的对象。两个对象都被转译成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 的数组。</p>
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<p>如同复制字符到临时数组，再从该数组到 <code>dest</code> 一般发生复制。</p>
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<p>若 <code>dest</code> 或 <code>src</code> 为<a href="cpp-language-pointer.html#.E6.8C.87.E9.92.88">非法或空指针</a>则行为未定义，即使 <code>count</code> 为零。</p>
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<p>若对象<a href="cpp-language-object.html#.E5.AD.90.E5.AF.B9.E8.B1.A1">潜在重叠</a>或非<a href="cpp-named_req-TriviallyCopyable.html"><span style="font-style:italic">可平凡复制</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(TriviallyCopyable)</span></a> ，则 <code>memmove</code> 的行为未指定，且<a rel="nofollow" class="external text" href="http://stackoverflow.com/questions/29777492">可能未定义</a>。</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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<tr class="t-par">
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<td>dest</td>
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<td>-</td>
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<td>指向复制目的的内存位置的指针</td>
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</tr>
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<tr class="t-par">
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<td>src</td>
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<td>-</td>
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<td>指向复制来源的内存位置的指针</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>要复制的字节数</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><code>dest</code></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><code>std::memmove</code> 可用于在目标缓冲区中<a href="cpp-language-object.html#.E5.AF.B9.E8.B1.A1.E5.88.9B.E5.BB.BA">隐式创建</a>对象。</p>
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<p>尽管说明了“如同”使用临时缓冲区，此函数的实际实现不会带来二次复制或额外内存的开销。常用方法（ glibc 和 bsd libc ）是若目标在源之前开始，则从缓冲区开始正向复制，否则从末尾反向复制，完全无重叠时回落到更高效的 <span class="t-lc"><a href="cpp-string-byte-memcpy.html">std::memcpy</a></span> 。</p>
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<p>在<a href="cpp-language-object.html#.E4.B8.A5.E6.A0.BC.E7.9A.84.E5.88.AB.E5.90.8D.E4.BD.BF.E7.94.A8">严格别名使用</a>禁止检验同一内存为二个不同类型的值处，可用 <code>std::memmove</code> 转换值。</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">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> str<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"1234567890"</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> str <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">memmove</span><span class="br0">(</span>str <span class="sy2">+</span> <span class="nu0">4</span>, str <span class="sy2">+</span> <span class="nu0">3</span>, <span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 从 [4, 5, 6] 复制到 [5, 6, 7]</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> str <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">1234567890
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1234456890</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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<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-string-byte-memcpy.html"><span class="t-lines"><span>memcpy</span></span></a></div>
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</div>
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</td>
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<td>复制一个缓冲区到另一个<br />
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<span class="t-mark">(函数)</span></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-string-byte-memset.html"><span class="t-lines"><span>memset</span></span></a></div>
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</div>
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</td>
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<td>以一个字符填充缓冲区<br />
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<span class="t-mark">(函数)</span></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-string-wide-wmemmove.html"><span class="t-lines"><span>wmemmove</span></span></a></div>
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</div>
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</td>
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<td>在两个可能重叠的数组间复制一定量宽字符<br />
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<span class="t-mark">(函数)</span></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-algorithm-copy.html"><span class="t-lines"><span>copy</span><span>copy_if</span></span></a></div>
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<div><span class="t-lines"><span></span><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>将某一范围的元素复制到一个新的位置<br />
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<span class="t-mark">(函数模板)</span></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-algorithm-copy_backward.html"><span class="t-lines"><span>copy_backward</span></span></a></div>
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</div>
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</td>
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<td>按从后往前的顺序复制一个范围内的元素<br />
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<span class="t-mark">(函数模板)</span></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-types-is_trivially_copyable.html"><span class="t-lines"><span>is_trivially_copyable</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>检查类型是否可平凡复制<br />
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<span class="t-mark">(类模板)</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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</div>
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</div>
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</div>
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</body>
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</html>
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