430 lines
47 KiB
HTML
430 lines
47 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::hash</title>
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_hash skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>hash</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-utility.html">utility</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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<div>定义于头文件 <code><a href="cpp-header-functional.html"><functional></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> Key <span class="sy1">></span><br />
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<span class="kw1">struct</span> hash<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p>此模板的每个特化为<i>启用</i>（“无污染”）或为<i>禁用</i>（“中毒”）。</p>
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<p><code>hash</code> 函数模板的<i>启用</i>的特化定义一个实现<a href="https://en.wikipedia.org/wiki/Hash_function" class="extiw" title="enwiki:Hash function">散列函数</a>的函数对象。此函数对象的实例满足<a href="cpp-named_req-Hash.html"><span style="font-style:italic">散列</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Hash)</span></a> 。特别是它们定义满足下列条件的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">(</span><span class="br0">)</span></span></span> ：</p>
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<ol>
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<li>接收 <code>Key</code> 类型的单个参数</li>
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<li>返回表示参数散列值的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a></span></span> 类型。</li>
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<li>调用时不抛出异常。</li>
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<li>对于二个相等的参数 <code>k1</code> 与 <code>k2</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k1<span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k2<span class="br0">)</span></span></span> 。</li>
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<li>对于二个相异而不相等的参数 <code>k1</code> 与 <code>k2</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k1<span class="br0">)</span> <span class="sy1">==</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k2<span class="br0">)</span></span></span> 的概率应非常小，接近 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu16">1.0</span><span class="sy2">/</span><a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">></span><span class="sy4">::</span><span class="me2">max</span><span class="br0">(</span><span class="br0">)</span></span></span> 。</li>
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</ol>
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<p>库函数提供的 <code>hash</code> 的所有显式和部分特化<a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a> 、<a href="cpp-named_req-CopyAssignable.html"><span style="font-style:italic">可复制赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyAssignable)</span></a> 、<a href="cpp-named_req-Swappable.html"><span style="font-style:italic">可交换</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Swappable)</span></a> 且<a href="cpp-named_req-Destructible.html"><span style="font-style:italic">可析构</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Destructible)</span></a> 。用户提供的 <code>hash</code> 特化亦必须满足这些要求。</p>
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<p>无序关联容器 <span class="t-lc"><a href="cpp-container-unordered_set.html">std::unordered_set</a></span> 、 <span class="t-lc"><a href="cpp-container-unordered_multiset.html">std::unordered_multiset</a></span> 、 <span class="t-lc"><a href="cpp-container-unordered_map.html">std::unordered_map</a></span> 、 <span class="t-lc"><a href="cpp-container-unordered_multimap.html">std::unordered_multimap</a></span> 以该模板 <code>std::hash</code> 的特化为默认散列函数。</p>
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<p>对于每个既非库亦非用户提供的数据类型的 <code>Key</code> 启用特化的函数 <code>std::hash<Key></code>，特化存在且被禁用。被禁用特化不满足<a href="cpp-named_req-Hash.html"><span style="font-style:italic">散列</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Hash)</span></a> ，不满足<a href="cpp-named_req-FunctionObject.html"><span style="font-style:italic">函数对象</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(FunctionObject)</span></a> ，且下列值全为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ：</p>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_default_constructible.html"><span class="kw526">std::<span class="me2">is_default_constructible</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw532">std::<span class="me2">is_copy_constructible</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_constructible.html"><span class="kw538">std::<span class="me2">is_move_constructible</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_assignable.html"><span class="kw550">std::<span class="me2">is_copy_assignable</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_assignable.html"><span class="kw556">std::<span class="me2">is_move_assignable</span></span></a><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>Key<span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span></span></span></li>
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</ul>
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<p>换言之，它们存在但无法使用。</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>除了上述指定的情况外，散列函数的实现方式是实现定义的。（比如， gcc 的<a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B11/hashtable_c%2B%2B0x.cc">实现</a>）。需要注意，某些散列函数的实现过于简单，甚至是直接将对象映射至自身。这可能会造成严重的后果。换言之，这些散列函数可以用于无序关联容器，但它们不一定安全/抗碰撞。</p>
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<p>散列函数仅要求在程序的单次执行中对同样的输入返回同样的结果；这允许避免碰撞拒绝服务攻击的加盐哈希。</p>
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<p>没有对 C 字符串的特化。 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span><span class="sy1">></span></span></span> 产生指针值（内存地址）的散列值，它不检验任何字符数组的内容。</p>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx20">
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<table class="t-dsc-begin">
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<td>成员类型</td>
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<td>定义</td>
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<td><code>argument_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span></td>
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<td><code>Key</code></td>
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<tr class="t-dsc">
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<td><code>result_type</code><span class="t-mark-rev t-deprecated-cxx17">(C++17 中弃用)</span></td>
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<td><span class="t-lc"><a href="cpp-types-size_t.html">std::size_t</a></span></td>
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</tr>
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</table>
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</td>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</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 t-dsc-member-nobold-div">
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<div><a href="cpp-utility-hash-hash.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造散列函数对象<br />
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<span class="t-mark">(公开成员函数)</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-utility-hash-operator().html"><span class="t-lines"><span>operator()</span></span></a></div>
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</div>
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</td>
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<td>计算参数的散列值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</table>
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<h3><span class="mw-headline" id=".E5.9F.BA.E6.9C.AC.E7.B1.BB.E5.9E.8B.E7.9A.84.E6.A0.87.E5.87.86.E7.89.B9.E5.8C.96">基本类型的标准特化</span></h3>
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<div>定义于头文件 <code><a href="cpp-header-functional.html"><functional></a></code></div>
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</td>
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<td></td>
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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><span class="kw4">bool</span><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><span class="kw4">char</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><span class="kw4">signed</span> <span class="kw4">char</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><span class="kw4">unsigned</span> <span class="kw4">char</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>char8_t<span class="sy1">></span><span class="sy4">;</span> <span class="co1">// C++20</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><span class="kw4">char16_t</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><span class="kw4">char32_t</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><span class="kw4">wchar_t</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><span class="kw4">short</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><span class="kw4">unsigned</span> <span class="kw4">short</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><span class="kw4">int</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><span class="kw4">unsigned</span> <span class="kw4">int</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><span class="kw4">long</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><span class="kw4">long</span> <span class="kw4">long</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><span class="kw4">unsigned</span> <span class="kw4">long</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><span class="kw4">unsigned</span> <span class="kw4">long</span> <span class="kw4">long</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><span class="kw4">float</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><span class="kw4">double</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><span class="kw4">long</span> <span class="kw4">double</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><a href="cpp-types-nullptr_t.html"><span class="kw102">std::<span class="me2">nullptr_t</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">class</span> T <span class="sy1">></span> <span class="kw1">struct</span> hash<span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span><span class="sy4">;</span></div>
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</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>在上述之外，标准库对所有（有作用域或无作用域）枚举类型提供特化。可以（但不要求）实现为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span><a href="cpp-types-underlying_type.html"><span class="kw637">std::<span class="me2">underlying_type</span></span></a><span class="sy1"><</span>Enum<span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span><span class="sy1">></span></span></span> 。</p>
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<p><br />
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标准库提供 <code>std::hash</code> 对 <span class="t-lc"><a href="cpp-types-nullptr_t.html">std::nullptr_t</a></span> 和所有无 cv 限定算术类型（包含任何扩展整数类型）、所有枚举类型和所有指针类型的启用特化。</p>
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<p>每个声明模板 <code>std::hash</code> 标准库头文件提供上述启用特化。这些头文件包括 <a href="cpp-header-string.html"><tt><string></tt></a>、 <a href="cpp-header-system_error.html"><tt><system_error></tt></a>、 <a href="cpp-header-bitset.html"><tt><bitset></tt></a>、 <a href="cpp-header-memory.html"><tt><memory></tt></a>、 <a href="cpp-header-typeindex.html"><tt><typeindex></tt></a>、 <a href="cpp-header-vector.html"><tt><vector></tt></a>、 <a href="cpp-header-thread.html"><tt><thread></tt></a><span class="t-rev-inl t-since-cxx17"><span>、 <a href="cpp-header-optional.html"><tt><optional></tt></a> 、 <a href="cpp-header-variant.html"><tt><variant></tt></a>、 <a href="cpp-header-string_view.html"><tt><string_view></tt></a></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span><span class="t-rev-inl t-since-cxx20"><span>、 <a href="cpp-header-coroutine.html"><tt><coroutine></tt></a></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span><span class="t-rev-inl t-since-cxx23"><span>、 <a href="cpp-header-stacktrace.html"><tt><stacktrace></tt></a></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> 。</p>
|
|
<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx17">
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<td>
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<p>此模板的标准库特化的所有成员函数均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span></span></span> ，除了 <a href="cpp-utility-optional-hash.html"><tt>std::hash<std::optional></tt></a> 、 <a href="cpp-utility-variant-hash.html"><tt>std::hash<std::variant></tt></a> 和 <a href="cpp-memory-unique_ptr-hash.html"><tt>std::hash<std::unique_ptr></tt></a> 的成员函数。</p>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</table>
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|
<h3><span class="mw-headline" id=".E5.BA.93.E7.B1.BB.E5.9E.8B.E7.9A.84.E6.A0.87.E5.87.86.E7.89.B9.E5.8C.96">库类型的标准特化</span></h3>
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<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-string-basic_string-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::string></span></span><span>std::hash<span class="t-dsc-small"><std::u8string></span></span><span>std::hash<span class="t-dsc-small"><std::u16string></span></span><span>std::hash<span class="t-dsc-small"><std::u32string></span></span><span>std::hash<span class="t-dsc-small"><std::wstring></span></span><span>std::hash<span class="t-dsc-small"><std::pmr::string></span></span><span>std::hash<span class="t-dsc-small"><std::pmr::u8string></span></span><span>std::hash<span class="t-dsc-small"><std::pmr::u16string></span></span><span>std::hash<span class="t-dsc-small"><std::pmr::u32string></span></span><span>std::hash<span class="t-dsc-small"><std::pmr::wstring></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</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><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 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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<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-error-error_code-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::error_code></span></span></span></a></div>
|
|
<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><span class="t-lc"><a href="cpp-error-error_code.html">std::error_code</a></span> 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>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-bitset-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::bitset></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc"><a href="cpp-utility-bitset.html">std::bitset</a></span> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-memory-unique_ptr-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::unique_ptr></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc"><a href="cpp-memory-unique_ptr.html">std::unique_ptr</a></span> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
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|
<div><a href="cpp-memory-shared_ptr-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::shared_ptr></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-types-type_index-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::type_index></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc"><a href="cpp-types-type_index.html">std::type_index</a></span> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-container-vector_bool-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::vector<bool>></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><a href="cpp-container-vector_bool.html"><tt>std::vector<bool></tt></a> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-thread-thread-id-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::thread::id></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc"><a href="cpp-thread-thread-id.html">std::thread::id</a></span> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-optional-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::optional></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>特化 <span class="t-lc"><strong class="selflink">std::hash</strong></span> 算法<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-variant-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::variant></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>特化 <span class="t-lc"><strong class="selflink">std::hash</strong></span> 算法<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<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>
|
|
<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>
|
|
</div>
|
|
</td>
|
|
<td>string_view 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-error-error_condition-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::error_condition></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc"><a href="cpp-error-error_condition.html">std::error_condition</a></span> 的哈希支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-coroutine-coroutine_handle-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::coroutine_handle></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc">std::coroutine_handle</span> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-basic_stacktrace-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::basic_stacktrace></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc">std::basic_stacktrace</span> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-utility-stacktrace_entry-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::stacktrace_entry></span></span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-lc">std::stacktrace_entry</span> 的散列支持<br />
|
|
<span class="t-mark">(类模板特化)</span></td>
|
|
</tr>
|
|
</table>
|
|
<p>注意：对 <code>std::pair</code> 和标准容器类型的特化，还有组合哈希的工具函数可用于 <a rel="nofollow" class="external text" href="http://www.boost.org/doc/libs/release/doc/html/hash/reference.html">boost.hash</a></p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <iostream></span>
|
|
<span class="co2">#include <iomanip></span>
|
|
<span class="co2">#include <functional></span>
|
|
<span class="co2">#include <string></span>
|
|
<span class="co2">#include <unordered_set></span>
|
|
|
|
<span class="kw1">struct</span> S <span class="br0">{</span>
|
|
<a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> first_name<span class="sy4">;</span>
|
|
<a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> last_name<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> S<span class="sy3">&</span> lhs, <span class="kw4">const</span> S<span class="sy3">&</span> rhs<span class="br0">)</span> <span class="br0">{</span>
|
|
<span class="kw1">return</span> lhs.<span class="me1">first_name</span> <span class="sy1">==</span> rhs.<span class="me1">first_name</span> <span class="sy3">&&</span> lhs.<span class="me1">last_name</span> <span class="sy1">==</span> rhs.<span class="me1">last_name</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="co1">// 自定义散列函数能是独立函数对象：</span>
|
|
<span class="kw1">struct</span> MyHash
|
|
<span class="br0">{</span>
|
|
<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>S <span class="kw4">const</span><span class="sy3">&</span> s<span class="br0">)</span> <span class="kw4">const</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> h1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">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="me1">first_name</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> h2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">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="me1">last_name</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw1">return</span> h1 <span class="sy3">^</span> <span class="br0">(</span>h2 <span class="sy1"><<</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 或使用 boost::hash_combine （见讨论）</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="co1">// std::hash 的自定义特化能注入 namespace std</span>
|
|
<span class="kw1">namespace</span> std
|
|
<span class="br0">{</span>
|
|
<span class="kw1">template</span><span class="sy1"><></span> <span class="kw1">struct</span> hash<span class="sy1"><</span>S<span class="sy1">></span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">typedef</span> S argument_type<span class="sy4">;</span>
|
|
<span class="kw1">typedef</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> result_type<span class="sy4">;</span>
|
|
result_type operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>argument_type <span class="kw4">const</span><span class="sy3">&</span> s<span class="br0">)</span> <span class="kw4">const</span>
|
|
<span class="br0">{</span>
|
|
result_type <span class="kw4">const</span> h1 <span class="br0">(</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">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="me1">first_name</span><span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span>
|
|
result_type <span class="kw4">const</span> h2 <span class="br0">(</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">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="me1">last_name</span><span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw1">return</span> h1 <span class="sy3">^</span> <span class="br0">(</span>h2 <span class="sy1"><<</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 或使用 boost::hash_combine （见讨论）</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<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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<a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"Meet the new boss..."</span><span class="sy4">;</span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> str_hash <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1229">std::<span class="me2">string</span></span></a><span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>str<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">"hash("</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>str<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">") = "</span> <span class="sy1"><<</span> str_hash <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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S obj <span class="sy1">=</span> <span class="br0">{</span> <span class="st0">"Hubert"</span>, <span class="st0">"Farnsworth"</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// 使用独立的函数对象</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">"hash("</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>obj.<span class="me1">first_name</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">", "</span>
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<span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>obj.<span class="me1">last_name</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">") = "</span>
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<span class="sy1"><<</span> MyHash<span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>obj<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" (using MyHash)<span class="es1">\n</span>"</span> <span class="sy1"><<</span> <a href="cpp-io-manip-setw.html"><span class="kw1796">std::<span class="me2">setw</span></span></a><span class="br0">(</span><span class="nu0">31</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"or "</span>
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<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">hash</span><span class="sy1"><</span>S<span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>obj<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" (using injected std::hash<S> specialization)<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co1">// 自定义散列函数令在无序容器中使用自定义类型可行</span>
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<span class="co1">// 此示例将使用注入的 std::hash 特化，</span>
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<span class="co1">// 若要使用 MyHash 替代，则将其作为第二模板参数传递</span>
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<a href="cpp-container-unordered_set.html"><span class="kw1288">std::<span class="me2">unordered_set</span></span></a><span class="sy1"><</span>S<span class="sy1">></span> names <span class="sy1">=</span> <span class="br0">{</span>obj, <span class="br0">{</span><span class="st0">"Bender"</span>, <span class="st0">"Rodriguez"</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">"Leela"</span>, <span class="st0">"Turanga"</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&</span> s<span class="sy4">:</span> names<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-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>s.<span class="me1">first_name</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1801">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>s.<span class="me1">last_name</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></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">hash("Meet the new boss...") = 1861821886482076440
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hash("Hubert", "Farnsworth") = 17622465712001802105 (using MyHash)
|
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or 17622465712001802105 (using injected std::hash<S> specialization)
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"Turanga" "Leela"
|
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"Bender" "Rodriguez"
|
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"Hubert" "Farnsworth"</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/issue2148">LWG 2148</a></td>
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<td>C++11</td>
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<td>缺失对枚举的特化</td>
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<td>已提供</td>
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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/issue2543">LWG 2543</a></td>
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<td>C++11</td>
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<td><code>hash</code> 可能不是 SFINAE 友好的</td>
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<td>通过禁用的特化使之为 SFINAE 友好</td>
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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/issue2817">LWG 2817</a></td>
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<td>C++11</td>
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<td>缺失对 <code>nullptr_t</code> 的特化</td>
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<td>已提供</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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