151 lines
12 KiB
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151 lines
12 KiB
HTML
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<title>std::weak_ptr<T>::weak_ptr</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_weak_ptr_weak_ptr skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::weak_ptr<T>::</span>weak_ptr</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-memory.html">memory</a>‎ | <a href="cpp-memory-weak_ptr.html">weak ptr</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> weak_ptr<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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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">weak_ptr<span class="br0">(</span> <span class="kw4">const</span> weak_ptr<span class="sy3">&</span> r <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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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 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> Y <span class="sy1">></span><br />
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weak_ptr<span class="br0">(</span> <span class="kw4">const</span> weak_ptr<span class="sy1"><</span>Y<span class="sy1">></span><span class="sy3">&</span> r <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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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 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> Y <span class="sy1">></span><br />
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weak_ptr<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>Y<span class="sy1">></span><span class="sy3">&</span> r <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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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 class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp">weak_ptr<span class="br0">(</span> weak_ptr<span class="sy3">&&</span> r <span class="br0">)</span> <span class="kw1">noexcept</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>
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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> Y <span class="sy1">></span><br />
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weak_ptr<span class="br0">(</span> weak_ptr<span class="sy1"><</span>Y<span class="sy1">></span><span class="sy3">&&</span> r <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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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>
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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>构造新的 <code>weak_ptr</code> ，潜在地与 <code>r</code> 共享对象的。</p>
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<div class="t-li1"><span class="t-li">1)</span> 默认构造函数。构造空 <code>weak_ptr</code> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 构造新的 <code>weak_ptr</code> ，它共享 <code>r</code> 所管理的对象。若 <code>r</code> 不管理对象，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 亦不管理对象。模板重载不参与重载决议，除非 <code>Y*</code> 可隐式转换为 <code>T*</code> <span class="t-rev-inl t-since-cxx17"><span>，或 <code>Y</code> 是某类型 <code>U</code> 和某数 <code>N</code> 的“ <code>N</code> 个 <code>U</code> 的数组”类型，而 <code>T</code> 是“（可有 cv 限定的） <code>U</code> 的未知边界数组”类型</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span>。</div>
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<div class="t-li1"><span class="t-li">3)</span> 移动构造函数。从 <code>r</code> 移动 weak_ptr 实例到 <code>*this</code> 。之后， <code>r</code> 为空且 <span class="t-c"><span class="mw-geshi cpp source-cpp">r.<span class="me1">use_count</span><span class="br0">(</span><span class="br0">)</span><span class="sy1">==</span><span class="nu0">0</span></span></span> 。模板重载不参与重载决议，除非 <code>Y*</code> 可隐式转换为 <code>T*</code> 。</div>
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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>r</td>
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<td>-</td>
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<td>将为此 <span class="t-lc"><a href="cpp-memory-weak_ptr.html">std::weak_ptr</a></span> 所查看的 <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></td>
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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>因为默认构造函数是 <code>constexpr</code> ，静态 <code>weak_ptr</code> 会在任何动态非局部初始化之前，作为<a href="cpp-language-initialization.html#.E9.9D.9E.E5.B1.80.E9.83.A8.E5.8F.98.E9.87.8F">静态非局部初始化</a>的一部分初始化。这使得在任何静态对象的构造函数中使用 <code>weak_ptr</code> 是安全的。</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 <memory></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> Foo <span class="br0">{</span><span class="br0">}</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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<a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span>Foo<span class="sy1">></span> w_ptr<span class="sy4">;</span>
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<span class="br0">{</span>
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<span class="kw4">auto</span> ptr <span class="sy1">=</span> <a href="cpp-memory-shared_ptr-make_shared.html"><span class="kw736">std::<span class="me2">make_shared</span></span></a><span class="sy1"><</span>Foo<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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w_ptr <span class="sy1">=</span> ptr<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> <span class="st0">"w_ptr.use_count() inside scope: "</span> <span class="sy1"><<</span> w_ptr.<span class="me1">use_count</span><span class="br0">(</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>
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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> <span class="st0">"w_ptr.use_count() out of scope: "</span> <span class="sy1"><<</span> w_ptr.<span class="me1">use_count</span><span class="br0">(</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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"w_ptr.expired() out of scope: "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a> <span class="sy1"><<</span> w_ptr.<span class="me1">expired</span><span class="br0">(</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>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">w_ptr.use_count() inside scope: 1
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w_ptr.use_count() out of scope: 0
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w_ptr.expired() out of scope: true</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/issue2315">LWG 2315</a></td>
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<td>C++11</td>
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<td>移动语义未对 <code>weak_ptr</code> 启用</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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-weak_ptr-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>为<code>weak_ptr</code>赋值<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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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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