187 lines
16 KiB
HTML
187 lines
16 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::call_once</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_thread_call_once 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::</span>call_once</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-thread.html">thread</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-dsc-header">
|
|
<td>
|
|
<div>定义于头文件 <code><a href="cpp-header-mutex.html"><mutex></a></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx11">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> Callable, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
|
|
<span class="kw4">void</span> call_once<span class="br0">(</span> <a href="cpp-thread-once_flag.html"><span class="kw2174">std::<span class="me2">once_flag</span></span></a><span class="sy3">&</span> flag, Callable<span class="sy3">&&</span> f, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</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>准确执行一次<a href="cpp-named_req-Callable.html"><span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></a> 对象 <code>f</code> ，即使同时从多个线程调用。</p>
|
|
<p>细节为：</p>
|
|
<ul>
|
|
<li>若在调用 <code>call_once</code> 的时刻， <code>flag</code> 指示已经调用了 <code>f</code> ，则 <code>call_once</code> 立即返回（称这种对 <code>call_once</code> 的调用为<i>消极</i>）。</li>
|
|
</ul>
|
|
<ul>
|
|
<li>否则， <code>call_once</code> 以参数 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> 调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Callable<span class="sy1">></span><span class="br0">(</span>f<span class="br0">)</span></span></span> （如同用 <span class="t-lc"><a href="cpp-utility-functional-invoke.html">std::invoke</a></span> ）。不同于 <span class="t-lc"><a href="cpp-thread-thread.html">std::thread</a></span> 构造函数或 <span class="t-lc"><a href="cpp-thread-async.html">std::async</a></span> ，不移动或复制参数，因为不需要转移它们到另一执行线程（称这种对 <code>call_once</code> 的调用为<i>积极</i>）。</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>若该调用抛异常，则传播异常给 <code>call_once</code> 的调用方，并且不翻转 <code>flag</code> ，以令其他调用将得到尝试（称这种对 <code>call_once</code> 的调用为<i>异常</i>）。</li>
|
|
<li>若该调用正常返回（称这种对 <code>call_once</code> 的调用为<i>返回</i>），则翻转 <code>flag</code> ，并保证以同一 <code>flag</code> 对 <code>call_once</code> 的其他调用为<i>消极</i>。</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<p>同一 <code>flag</code> 上的所有<i>积极</i>调用组成单独全序，它们由零或多个<i>异常</i>调用后随一个<i>返回</i>调用组成。该顺序中，每个<i>积极</i>调用的结尾同步于下个<i>积极</i>调用。</p>
|
|
<p>从<i>返回</i>调用的返回同步于同一 <code>flag</code> 上的所有<i>消极</i>调用：这表示保证所有对 <code>call_once</code> 的同时调用都观察到<i>积极</i>调用所做的任何副效应，而无需额外同步。</p>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
|
<table class="t-par-begin">
|
|
<tr class="t-par">
|
|
<td>flag</td>
|
|
<td>-</td>
|
|
<td>对象，对于它只有一个函数得到执行</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>f</td>
|
|
<td>-</td>
|
|
<td>要调用的<a href="cpp-named_req-Callable.html"><span style="font-style:italic">可调用</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Callable)</span></a> 对象</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>args...</td>
|
|
<td>-</td>
|
|
<td>传递给函数的参数</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
|
<p>（无）</p>
|
|
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
|
|
<ul>
|
|
<li>若任何条件阻止对 <code>call_once</code> 的调用按规定执行，则抛出 <span class="t-lc"><a href="cpp-error-system_error.html">std::system_error</a></span></li>
|
|
<li>任何 <code>f</code> 所抛的异常</li>
|
|
</ul>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p>若对 <code>call_once</code> 的同时调用传递不同的 <code>f</code> ，则调用哪个 <code>f</code> 是未指定的。被选择函数运行于与传递它的 <code>call_once</code> 的调用相同的线程。</p>
|
|
<p>即使在从多个线程调用时，也保证<a href="cpp-language-storage_duration.html#.E9.9D.99.E6.80.81.E5.B1.80.E9.83.A8.E5.8F.98.E9.87.8F">函数局域静态对象</a>的初始化仅出现一次，这可能比使用 <code>std::call_once</code> 的等价代码更为高效。</p>
|
|
<p>此函数的 POSIX 类似物是 <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_once.html"><code>pthread_once</code></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 <thread></span>
|
|
<span class="co2">#include <mutex></span>
|
|
|
|
<a href="cpp-thread-once_flag.html"><span class="kw2174">std::<span class="me2">once_flag</span></span></a> flag1, flag2<span class="sy4">;</span>
|
|
|
|
<span class="kw4">void</span> simple_do_once<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
std<span class="sy4">::</span><span class="me2">call_once</span><span class="br0">(</span>flag1, <span class="br0">[</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">"Simple example: called once<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">void</span> may_throw_function<span class="br0">(</span><span class="kw4">bool</span> do_throw<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">if</span> <span class="br0">(</span>do_throw<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">"throw: call_once will retry<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// 这会出现多于一次</span>
|
|
<span class="kw1">throw</span> <a href="cpp-error-exception.html"><span class="kw764">std::<span class="me2">exception</span></span></a><span class="br0">(</span><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">"Didn't throw, call_once will not attempt again<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// 保证一次</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">void</span> do_once<span class="br0">(</span><span class="kw4">bool</span> do_throw<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw1">try</span> <span class="br0">{</span>
|
|
std<span class="sy4">::</span><span class="me2">call_once</span><span class="br0">(</span>flag2, may_throw_function, do_throw<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
<span class="kw1">catch</span> <span class="br0">(</span>...<span class="br0">)</span> <span class="br0">{</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-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> st1<span class="br0">(</span>simple_do_once<span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> st2<span class="br0">(</span>simple_do_once<span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> st3<span class="br0">(</span>simple_do_once<span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> st4<span class="br0">(</span>simple_do_once<span class="br0">)</span><span class="sy4">;</span>
|
|
st1.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
st2.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
st3.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
st4.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t1<span class="br0">(</span>do_once, <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t2<span class="br0">(</span>do_once, <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t3<span class="br0">(</span>do_once, <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="cpp-thread-thread.html"><span class="kw2146">std::<span class="me2">thread</span></span></a> t4<span class="br0">(</span>do_once, <span class="kw2">true</span><span class="br0">)</span><span class="sy4">;</span>
|
|
t1.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
t2.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
t3.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
t4.<span class="me1">join</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>可能的输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">Simple example: called once
|
|
throw: call_once will retry
|
|
throw: call_once will retry
|
|
Didn't throw, call_once will not attempt again</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/issue2442">LWG 2442</a></td>
|
|
<td>C++11</td>
|
|
<td>在调用前复制并/或移动参数</td>
|
|
<td>不进行复制/移动</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-thread-once_flag.html"><span class="t-lines"><span>once_flag</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>确保 <strong class="selflink"><tt>call_once</tt></strong> 只调用函数一次的帮助对象<br />
|
|
<span class="t-mark">(类)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|