285 lines
19 KiB
HTML
285 lines
19 KiB
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::owner_less</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_owner_less 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>owner_less</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-memory.html">memory</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-memory.html"><memory></a></code></div>
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<td rowspan="142">(1)</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">class</span> T <span class="sy1">></span><br />
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<span class="kw1">struct</span> owner_less<span class="sy4">;</span> <span class="coMULTI">/* undefined */</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span><br />
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<span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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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> T <span class="sy1">=</span> <span class="kw4">void</span> <span class="sy1">></span><br />
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<span class="kw1">struct</span> owner_less<span class="sy4">;</span> <span class="coMULTI">/* undefined */</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-cxx17">(C++17 起)</span></td>
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</tbody>
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<tbody>
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<tr class="t-dcl t-since-cxx11">
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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> T <span class="sy1">></span><br />
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<span class="kw1">struct</span> owner_less<span class="sy1"><</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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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> T <span class="sy1">></span><br />
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<span class="kw1">struct</span> owner_less<span class="sy1"><</span><a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span><span class="sy4">;</span></span></div>
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</td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<tr class="t-dcl t-since-cxx17">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><></span><br />
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<span class="kw1">struct</span> owner_less<span class="sy1"><</span><span class="kw4">void</span><span class="sy1">></span><span class="sy4">;</span></span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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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>此函数对象提供基于拥有者（不同于基于值）的， <span class="t-lc"><a href="cpp-memory-weak_ptr.html">std::weak_ptr</a></span> 和 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 两者的混合类型序。顺序满足二个智能指针比较相等，若且唯若它们均为空或共享所有权，即使由 <code>get()</code> 获得的裸指针值相异（例如因为它们指向同一对象中的不同子对象）。</p>
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<p>在以 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 或 <span class="t-lc"><a href="cpp-memory-weak_ptr.html">std::weak_ptr</a></span> 为关键建立关联容器，即</p>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-container-map.html"><span class="kw1280">std::<span class="me2">map</span></span></a><span class="sy1"><</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span>, U, std<span class="sy4">::</span><span class="me2">owner_less</span><span class="sy1"><</span><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">>>></span></span></span></p>
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<p>或</p>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-container-map.html"><span class="kw1280">std::<span class="me2">map</span></span></a><span class="sy1"><</span><a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span>, U, std<span class="sy4">::</span><span class="me2">owner_less</span><span class="sy1"><</span><a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">>>></span></span></span> 时，最好使用此类模板。</p>
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<p>默认的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy1"><</span></span></span> 不为弱指针定义，并且可能错误地认为同一对象的二个共享指针不等价（见 <a href="cpp-memory-shared_ptr-owner_before.html"><tt>shared_ptr::owner_before</tt></a> ）。</p>
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<h3><span class="mw-headline" id=".E7.89.B9.E5.8C.96">特化</span></h3>
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<table class="t-rev-begin">
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<p>标准库提供 <code>std::owner_less</code> 在不指定 <code>T</code> 时的特化。此情况下，从实参类型推导形参类型（每个实参都必须是 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 或 <span class="t-lc"><a href="cpp-memory-weak_ptr.html">std::weak_ptr</a></span> ）。</p>
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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-memory-owner_less_void.html"><span class="t-lines"><span>owner_less<span class="t-dsc-small"><void></span></span></span></a></div>
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</div>
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</td>
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<td>为共享指针和弱指针提供混合类型的、基于拥有者的顺序的函数对象，无关乎被指向的类型<br />
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<span class="t-mark">(类模板特化)</span></td>
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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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</tr>
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</table>
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<table class="t-rev-begin">
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<tr class="t-rev t-until-cxx20">
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<td>
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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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<tr class="t-dsc-hitem">
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<td>成员类型</td>
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<td>定义</td>
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<tr class="t-dsc">
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<td><code>result_type</code><span class="t-mark">(C++17 中弃用)</span></td>
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<td>2-3) <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span></td>
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</tr>
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<tr class="t-dsc">
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<td><code>first_argument_type</code><span class="t-mark">(C++17 中弃用)</span></td>
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<td>2) <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span><br />
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3) <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span></td>
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</tr>
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<tr class="t-dsc">
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<td><code>second_argument_type</code><span class="t-mark">(C++17 中弃用)</span></td>
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<td>2) <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span><br />
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3) <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></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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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><strong class="selflink"><span class="t-lines"><span>operator()</span></span></strong></div>
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</div>
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</td>
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<td>用基于拥有者的语义比较其参数<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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<div class="t-member">
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<h2><span class="mw-headline" id="std::owner_less::operator.28.29"><span style="font-size:0.7em; line-height:130%">std::owner_less::</span>operator()</span></h2>
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<table class="t-dcl-begin">
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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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</tr>
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<tr class="t-dcl-h">
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<td>
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<div>所有模板特化的成员</div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> lhs,<br />
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<span class="kw4">const</span> <a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> rhs <span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> lhs,<br />
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<span class="kw4">const</span> <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> rhs <span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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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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<tr class="t-dcl-h">
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<td>
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<div>仅为 <code>owner_less<shared_ptr<T>></code> 模板特化的成员</div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> lhs,<br />
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<span class="kw4">const</span> <a href="cpp-memory-shared_ptr.html"><span class="kw734">std::<span class="me2">shared_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> rhs <span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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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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</tr>
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<tr class="t-dcl-h">
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<td>
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<div>仅为 <code>owner_less<weak_ptr<T>></code> 模板特化的成员</div>
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</td>
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<td></td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx11">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> lhs,<br />
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<span class="kw4">const</span> <a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> rhs <span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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</tr>
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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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</tr>
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</tbody>
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</table>
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<p>用基于拥有者的语义比较 <code>lhs</code> 与 <code>rhs</code> 。等效于调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">lhs.<span class="me1">owner_before</span><span class="br0">(</span>rhs<span class="br0">)</span></span></span> 。</p>
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<p>顺序是严格弱序关系。</p>
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<p><code>lhs</code> 与 <code>rhs</code> 相等，当且仅当它们均为空或共享所有权。</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>lhs, rhs</td>
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<td>-</td>
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<td>要比较的共享所有权指针</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>若按基于拥有者的顺序确定 <code>lhs</code> <i>小于</i> <code>rhs</code> ，则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</p>
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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/issue2873">LWG 2873</a></td>
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<td>C++11</td>
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<td>operator() 可以不声明为 noexcept</td>
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<td>声明为 noexcept</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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<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-memory-shared_ptr-owner_before.html"><span class="t-lines"><span>owner_before</span></span></a></div>
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</div>
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</td>
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<td>提供基于拥有者的共享指针排序<br />
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<span class="t-mark">(<code>std::shared_ptr<T></code> 的公开成员函数)</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-memory-weak_ptr-owner_before.html"><span class="t-lines"><span>owner_before</span></span></a></div>
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</div>
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</td>
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<td>提供弱指针的基于拥有者顺序<br />
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<span class="t-mark">(<code>std::weak_ptr<T></code> 的公开成员函数)</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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