120 lines
9.8 KiB
HTML
120 lines
9.8 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>std::weak_ptr<T>::lock</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_weak_ptr_lock skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::weak_ptr<T>::</span>lock</h1>
|
|
<div id="bodyContent">
|
|
<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>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<table class="t-dcl-begin">
|
|
<tbody>
|
|
<tr class="t-dcl t-since-cxx11">
|
|
<td>
|
|
<div><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> lock<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p>创建新的 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 对象，它共享被管理对象的所有权。若无被管理对象，即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 为空，则返回亦为空的 <code>shared_ptr</code> 。</p>
|
|
<p>等效地返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">expired<span class="br0">(</span><span class="br0">)</span> <span class="sy4">?</span> shared_ptr<span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> shared_ptr<span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span></span></span> ，原子地执行。</p>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
|
<p>（无）</p>
|
|
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
|
<p>若 <span class="t-lc"><a href="cpp-memory-weak_ptr-expired.html">std::weak_ptr::expired</a></span> 返回 <code>false</code> 则为共享被占有对象所有权的 <code>shared_ptr</code> 。否则返回默认构造的 T 类型的 <code>shared_ptr</code> 。</p>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
|
|
<p>此函数和 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 的构造函数可能获得 <code>std::weak_ptr</code> 所指向的被管理对象的临时所有权。区别是 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 的构造函数在其 <code>std::weak_ptr</code> 为空时抛异常，而 <span class="t-lc">std::weak_ptr<T>::lock()</span> 构造空的 <span class="t-lc">std::shared_ptr<T></span> 。</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 <memory></span>
|
|
|
|
<span class="kw4">void</span> observe<span class="br0">(</span><a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> weak<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">if</span> <span class="br0">(</span><span class="kw4">auto</span> observe <span class="sy1">=</span> weak.<span class="me1">lock</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\t</span>observe() able to lock weak_ptr<>, value="</span> <span class="sy1"><<</span> <span class="sy2">*</span>observe <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\t</span>observe() unable to lock weak_ptr<><span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-memory-weak_ptr.html"><span class="kw741">std::<span class="me2">weak_ptr</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> weak<span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"weak_ptr<> not yet initialized<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
observe<span class="br0">(</span>weak<span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="br0">{</span>
|
|
<span class="kw4">auto</span> shared <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><span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span>
|
|
weak <span class="sy1">=</span> shared<span class="sy4">;</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"weak_ptr<> initialized with shared_ptr.<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
observe<span class="br0">(</span>weak<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"shared_ptr<> has been destructed due to scope exit.<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
observe<span class="br0">(</span>weak<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">weak_ptr<> not yet initialized
|
|
observe() unable to lock weak_ptr<>
|
|
weak_ptr<> initialized with shared_ptr.
|
|
observe() able to lock weak_ptr<>, value=42
|
|
shared_ptr<> has been destructed due to scope exit.
|
|
observe() unable to lock weak_ptr<></pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
|
<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>
|
|
<table class="dsctable" style="font-size:0.8em">
|
|
<tr>
|
|
<th>DR</th>
|
|
<th>应用于</th>
|
|
<th>出版时的行为</th>
|
|
<th>正确行为</th>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2316">LWG 2316</a></td>
|
|
<td>C++11</td>
|
|
<td>不要求 lock() 是原子的，但要求为 noexcept ，这导致矛盾</td>
|
|
<td>指定为原子的</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-memory-weak_ptr-expired.html"><span class="t-lines"><span>expired</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>检查被引用的对象是否已删除<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|