137 lines
13 KiB
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137 lines
13 KiB
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<title>std::endl</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_manip_endl 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::</span>endl</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-io.html">io</a>‎ | <a href="cpp-io-manip.html">manip</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-ostream.html"><ostream></a></code></div>
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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">class</span> CharT, <span class="kw1">class</span> Traits <span class="sy1">></span><br />
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<a href="cpp-io-basic_ostream.html"><span class="kw1715">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>CharT, Traits<span class="sy1">></span><span class="sy3">&</span> endl<span class="br0">(</span> <a href="cpp-io-basic_ostream.html"><span class="kw1715">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>CharT, Traits<span class="sy1">></span><span class="sy3">&</span> os <span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>插入换行符到输出序列 <code>os</code> 并冲入它，如同调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">os.<span class="me1">put</span><span class="br0">(</span>os.<span class="me1">widen</span><span class="br0">(</span><span class="st0">'<span class="es1">\n</span>'</span><span class="br0">)</span><span class="br0">)</span></span></span> 后随 <span class="t-c"><span class="mw-geshi cpp source-cpp">os.<span class="me1">flush</span><span class="br0">(</span><span class="br0">)</span></span></span> 。</p>
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<p>这是仅输出的 I/O 操纵符，可对任何 <span class="t-lc"><a href="cpp-io-basic_ostream.html">std::basic_ostream</a></span> 类型的 <code>out</code> 以表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">out <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">endl</span></span></span> 调用它。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>可用此操纵符立即产生新行的输出，例如在从长时间运行的进程显示输出、记录多个线程的活动或记录可能不期待地崩溃的程序活动时。在调用 <span class="t-lc"><a href="cpp-utility-program-system.html">std::system</a></span> 前亦需要 <span class="t-lc"><a href="cpp-io-cout.html">std::cout</a></span> 的显式冲入，若产生的进程进行任何屏幕 I/O 。多数其他通常的交互 I/O 场景中，使用 <span class="t-lc"><a href="cpp-io-cout.html">std::cout</a></span> 时 <code>std::endl</code> 是冗余的，因为任何来自 <span class="t-lc"><a href="cpp-io-cin.html">std::cin</a></span> 的输入、到 <span class="t-lc"><a href="cpp-io-cerr.html">std::cerr</a></span> 的输出或程序终止强制调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>.<span class="me1">flush</span><span class="br0">(</span><span class="br0">)</span></span></span> 。某些源代码中鼓励用 <code>std::endl</code> 代替 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">'<span class="es1">\n</span>'</span></span></span> ，这可能显著地降低输出性能。</p>
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<p>多数实现中，标准输出是行缓冲的，而写入 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">'<span class="es1">\n</span>'</span></span></span> 就会导致冲入，除非执行 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">ios</span><span class="sy4">::</span><span class="me2">sync_with_stdio</span><span class="br0">(</span><span class="kw2">false</span><span class="br0">)</span></span></span> 。这些情形中，不必要的 <code>endl</code> 只会降低文件输出的性能，而非标准输出的。</p>
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<p>此维基上的代码示例<a rel="nofollow" class="external text" href="http://www.stroustrup.com/3rd_code.html">遵循 Bjarne Stroustrup</a>和<a rel="nofollow" class="external text" href="https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rio-endl">《 C++ 核心方针》</a>，只在需要时冲入标准输出。</p>
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<p>需要冲入不完整的行时，可使用 <span class="t-lc"><a href="cpp-io-manip-flush.html">std::flush</a></span> 操纵符。</p>
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<p>需要冲入每个字节的输出时，可使用 <span class="t-lc"><a href="cpp-io-manip-unitbuf.html">std::unitbuf</a></span> 操纵符。</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>os</td>
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<td>-</td>
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<td>到输出流的引用</td>
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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><code>os</code> （到操纵后流的引用）</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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<p>以 \n 替代 endl ，输出会相同，但可能不会实时出现。</p>
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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 <iostream></span>
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<span class="co2">#include <chrono></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> Diff<span class="sy1">></span>
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<span class="kw4">void</span> log_progress<span class="br0">(</span>Diff d<span class="br0">)</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> <a href="cpp-chrono-duration-duration_cast.html"><span class="kw1000">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration_cast</span></span></a><span class="sy1"><</span><a href="cpp-chrono-duration.html"><span class="kw996">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">milliseconds</span></span></a><span class="sy1">></span><span class="br0">(</span>d<span class="br0">)</span>.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span>
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<span class="sy1"><<</span> <span class="st0">" ms passed"</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">endl</span><span class="sy4">;</span>
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<span class="br0">}</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-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>.<span class="me1">sync_with_stdio</span><span class="br0">(</span><span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 一些平台上 stdout 在写 \n 时冲入</span>
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<span class="kw4">volatile</span> <span class="kw4">int</span> sink <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw4">auto</span> t1 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</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">int</span> j<span class="sy1">=</span><span class="nu0">0</span><span class="sy4">;</span> j<span class="sy1"><</span><span class="nu0">5</span><span class="sy4">;</span> <span class="sy2">++</span>j<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> n<span class="sy1">=</span><span class="nu0">0</span><span class="sy4">;</span> n<span class="sy1"><</span><span class="nu0">10000</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> m<span class="sy1">=</span><span class="nu0">0</span><span class="sy4">;</span> m<span class="sy1"><</span><span class="nu0">20000</span><span class="sy4">;</span> <span class="sy2">++</span>m<span class="br0">)</span>
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sink <span class="sy2">+</span><span class="sy1">=</span> m<span class="sy2">*</span>n<span class="sy4">;</span> <span class="co1">// 做一些工作</span>
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<span class="kw4">auto</span> now <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">high_resolution_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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log_progress<span class="br0">(</span>now <span class="sy2">-</span> t1<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</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">487 ms passed
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974 ms passed
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1470 ms passed
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1965 ms passed
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2455 ms passed</pre></div>
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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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<div class="t-dsc-member-div">
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<div><a href="cpp-io-manip-unitbuf.html"><span class="t-lines"><span>unitbuf</span><span>nounitbuf</span></span></a></div>
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</div>
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</td>
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<td>控制是否每次操作后冲洗输出<br />
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<span class="t-mark">(函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-manip-flush.html"><span class="t-lines"><span>flush</span></span></a></div>
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</div>
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</td>
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<td>冲洗输出流<br />
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<span class="t-mark">(函数模板)</span></td>
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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-io-basic_ostream-flush.html"><span class="t-lines"><span>flush</span></span></a></div>
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</div>
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</td>
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<td>与底层存储设备同步<br />
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<span class="t-mark">(<code>std::basic_ostream<CharT,Traits></code> 的公开成员函数)</span></td>
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<div class="visualClear"></div>
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