147 lines
13 KiB
HTML
147 lines
13 KiB
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<title>std::hash (std::string, std::wstring, std::u16string, std::u32string, std::u8string, std::pmr::string, std::pmr::wstring, std::pmr::u16string, std::pmr::u32string, std::pmr::u8string)</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>hash <small>(std::string, std::wstring, std::u16string, std::u32string, std::u8string, std::pmr::string, std::pmr::wstring, std::pmr::u16string, std::pmr::u32string, std::pmr::u8string)</small></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-basic_string.html">basic string</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-string.html"><string></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span><span class="sy4">;</span><br /></span>
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<p><span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1232">std::<span class="me2">wstring</span></span></a><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1234">std::<span class="me2">u16string</span></span></a><span class="sy1">></span><span class="sy4">;</span><br /></p>
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<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1236">std::<span class="me2">u32string</span></span></a><span class="sy1">></span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1238">std::<span class="me2">pmr</span><span class="sy4">::</span><span class="me2">string</span></span></a><span class="sy1">></span><span class="sy4">;</span><br /></span>
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<p><span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1240">std::<span class="me2">pmr</span><span class="sy4">::</span><span class="me2">wstring</span></span></a><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1242">std::<span class="me2">pmr</span><span class="sy4">::</span><span class="me2">u16string</span></span></a><span class="sy1">></span><span class="sy4">;</span><br /></p>
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<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1244">std::<span class="me2">pmr</span><span class="sy4">::</span><span class="me2">u32string</span></span></a><span class="sy1">></span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">u8string</span><span class="sy1">></span><span class="sy4">;</span><br />
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<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">pmr</span><span class="sy4">::</span><span class="me2">u8string</span><span class="sy1">></span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p><span class="t-lc"><a href="cpp-utility-hash.html">std::hash</a></span> 对各种字符串类的模板特化允许用户获得字符串的哈希。</p>
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<p>这些哈希等于对应 <span class="t-lc"><a href="cpp-string-basic_string_view.html">std::basic_string_view</a></span> 类的哈希：若 <code>S</code> 是这些字符串类型之一， <code>SV</code> 是对应的字符串视图类型，而 <code>s</code> 是 <code>S</code> 类型的对象，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span>S<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>s<span class="br0">)</span> <span class="sy1">==</span> <a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span>SV<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>SV<span class="br0">(</span>s<span class="br0">)</span><span class="br0">)</span></span></span> 。</p>
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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=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<p>下列代码显示 string 上使用的散列函数的一种可能输出：</p>
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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 <string></span>
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<span class="co2">#include <string_view></span>
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<span class="co2">#include <functional></span>
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<span class="co2">#include <memory_resource></span>
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<span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</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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<span class="kw4">auto</span> sv <span class="sy1">=</span> <span class="st0">"Stand back! I've got jimmies!"</span>sv<span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="br0">(</span>sv<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1238">std::<span class="me2">pmr</span><span class="sy4">::</span><span class="me2">string</span></span></a> pmrs<span class="br0">(</span>sv<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 使用默认分配器</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> <a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a><span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>sv<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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>s<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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1238">std::<span class="me2">pmr</span><span class="sy4">::</span><span class="me2">string</span></span></a><span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>pmrs<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></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">3544599705012401047
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3544599705012401047
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3544599705012401047</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2978">LWG 2978</a></td>
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<td>C++17</td>
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<td>未启用 <span class="t-lc"><a href="cpp-string-basic_string.html">std::pmr::string</a></span> 与其类似物的散列支持</td>
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<td>已启用</td>
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</tr>
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</table>
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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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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-hash.html"><span class="t-lines"><span>hash</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>散列函数对象<br />
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<span class="t-mark">(类模板)</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-basic_string_view-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::string_view></span></span><span>std::hash<span class="t-dsc-small"><std::wstring_view></span></span><span>std::hash<span class="t-dsc-small"><std::u8string_view></span></span><span>std::hash<span class="t-dsc-small"><std::u16string_view></span></span><span>std::hash<span class="t-dsc-small"><std::u32string_view></span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>string_view 的散列支持<br />
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<span class="t-mark">(类模板特化)</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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