231 lines
17 KiB
HTML
231 lines
17 KiB
HTML
<!DOCTYPE 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::basic_istream<CharT,Traits>::sentry</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_basic_istream_sentry 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::basic_istream<CharT,Traits>::</span>sentry</h1>
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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-basic_istream.html">basic istream</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">class</span> sentry<span class="sy4">;</span></span></div>
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<p>类 <code>basic_istream::sentry</code> 的对象在每个执行输入（有格式和无格式）的 <span class="t-lc"><a href="cpp-io-basic_istream.html">std::basic_istream</a></span> 成员函数起始的块作用域构造。其构造函数准备输入流：检查流是否已在失败状态，冲入 tie() 过的输出流，除非设置 <code>noskipws</code> 标志否则跳过前导空白符，并若需要则进行其他实现定义任务。若需要，则在析构函数中进行所有清理，从而若输入过程中抛出异常则保证清理发生。</p>
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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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<td><code>traits_type</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">Traits</span></span></td>
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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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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><strong class="selflink"><span class="t-lines"><span>(构造函数)</span></span></strong></div>
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</div>
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</td>
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<td>构造 sentry 对象。在此完成所有准备任务。<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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<tr class="t-dsc">
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><strong class="selflink"><span class="t-lines"><span>(析构函数)</span></span></strong></div>
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</div>
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</td>
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<td>若需要，则在有格式输入或异常后终止化流对象<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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<div><span class="t-lines"><span>operator=</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[被删除]</span></span></span></div>
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</td>
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<td>不可复制赋值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div><strong class="selflink"><span class="t-lines"><span>operator bool</span></span></strong></div>
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</td>
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<td>检查流对象的准备是否成功<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 class="t-member">
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<h2><span class="mw-headline" id="std::basic_istream::sentry::sentry"><span style="font-size:0.7em; line-height:130%">std::basic_istream::sentry::</span>sentry</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span> sentry<span class="br0">(</span><a href="cpp-io-basic_istream.html"><span class="kw1713">std::<span class="me2">basic_istream</span></span></a><span class="sy1"><</span>CharT,Traits<span class="sy1">></span><span class="sy3">&</span> is, <span class="kw4">bool</span> noskipws <span class="sy1">=</span> <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<td></td>
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<td></td>
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<p>为有格式输入准备流。</p>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">good</span><span class="br0">(</span><span class="br0">)</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ，则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">setstate</span><span class="br0">(</span>failbit<span class="br0">)</span></span></span><span class="t-mark">(C++11 起)</span> 并返回。否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">tie</span><span class="br0">(</span><span class="br0">)</span></span></span> 不是空指针，则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">tie</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>flush<span class="br0">(</span><span class="br0">)</span></span></span> 将输出序列与外部流同步。若 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">tie</span><span class="br0">(</span><span class="br0">)</span></span></span> 的放置区为空则可抑制此调用。实现可以延迟到 <span class="t-c"><span class="mw-geshi cpp source-cpp">flush</span></span> 的调用，直至出现调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>underflow<span class="br0">(</span><span class="br0">)</span></span></span> 。若在销毁 sentry 对象前不出现这种调用，则它可被完全消除。</p>
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<p>若 <code>noskipws</code> 为零且 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">flags</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">&</span> ios_base<span class="sy4">::</span><span class="me2">skipws</span></span></span> 非零，则函数释出并舍弃所有空白符，直至下个可用字符不是空白符（由当前 <code>is</code> 中感染的本地环境确定）。若 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>sbumpc<span class="br0">(</span><span class="br0">)</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>sgetc<span class="br0">(</span><span class="br0">)</span></span></span> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">traits<span class="sy4">::</span><span class="me2">eof</span><span class="br0">(</span><span class="br0">)</span></span></span> ，则函数调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">setstate<span class="br0">(</span>failbit <span class="sy3">|</span> eofbit<span class="br0">)</span></span></span> （可能抛出 <span class="t-lc"><a href="cpp-io-ios_base-failure.html">std::ios_base::failure</a></span> ）。</p>
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<p>可以发生另外的实现定义准备，可能调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">setstate<span class="br0">(</span>failbit<span class="br0">)</span></span></span> （可能抛出 <span class="t-lc"><a href="cpp-io-ios_base-failure.html">std::ios_base::failure</a></span> ）。</p>
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<p>若准备完成后 <span class="t-c"><span class="mw-geshi cpp source-cpp">is.<span class="me1">good</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">true</span></span></span> ，则对 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator <span class="kw4">bool</span></span></span> 的任何后继调用将返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</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>is</td>
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<td>-</td>
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<td>要准备的文件流</td>
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</tr>
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<tr class="t-par">
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<td>noskipws</td>
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<td>-</td>
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<td>若不应跳过空白符则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span></td>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>若在跳过空白符时出现文件尾条件则为 <span class="t-lc"><a href="cpp-io-ios_base-failure.html">std::ios_base::failure</a></span> 。</p>
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</div>
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<p><br /></p>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::basic_istream::sentry::.7Esentry"><span style="font-size:0.7em; line-height:130%">std::basic_istream::sentry::</span>~sentry</span></h2>
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<td class="t-dcl-nopad">
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<div><span class="mw-geshi cpp source-cpp">~sentry<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<td></td>
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<p>不做任何事。</p>
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</div>
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<p><br /></p>
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<div class="t-member">
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<h2><span class="mw-headline" id="std::basic_istream::sentry::operator_bool"><span style="font-size:0.7em; line-height:130%">std::basic_istream::sentry::</span>operator bool</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<p>检查输入流准备是否成功。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0_2">参数</span></h3>
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<p>（无）</p>
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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>若输入流初始化成功则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</p>
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</div>
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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 <iostream></span>
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<span class="co2">#include <sstream></span>
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<span class="kw1">struct</span> Foo
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<span class="br0">{</span>
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<span class="kw4">char</span> n<span class="br0">[</span><span class="nu0">5</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-io-basic_istream.html"><span class="kw1726">std::<span class="me2">istream</span></span></a><span class="sy3">&</span> operator<span class="sy1">>></span><span class="br0">(</span><a href="cpp-io-basic_istream.html"><span class="kw1726">std::<span class="me2">istream</span></span></a><span class="sy3">&</span> is, Foo<span class="sy3">&</span> f<span class="br0">)</span>
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<span class="br0">{</span>
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std<span class="sy4">::</span><span class="me2">istream</span><span class="sy4">::</span><span class="me2">sentry</span> s<span class="br0">(</span>is<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>s<span class="br0">)</span>
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is.<span class="me1">read</span><span class="br0">(</span>f.<span class="me1">n</span>, <span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> is<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-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> input <span class="sy1">=</span> <span class="st0">" abcde"</span><span class="sy4">;</span>
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<a href="cpp-io-basic_istringstream.html"><span class="kw1733">std::<span class="me2">istringstream</span></span></a> stream<span class="br0">(</span>input<span class="br0">)</span><span class="sy4">;</span>
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Foo f<span class="sy4">;</span>
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stream <span class="sy1">>></span> f<span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1755">std::<span class="me2">cout</span></span></a>.<span class="me1">write</span><span class="br0">(</span>f.<span class="me1">n</span>, <span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1755">std::<span class="me2">cout</span></span></a> <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">abcde</pre></div>
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</div>
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</div>
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<p><br /></p>
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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-io-basic_istream-operator_gtgt.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>提取带格式数据<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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<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_istream-operator_gtgt2.html"><span class="t-lines"><span>operator>><span class="t-dsc-small">(std::basic_istream)</span></span></span></a></div>
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</div>
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</td>
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<td>提取字符和字符数组<br />
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<span class="t-mark">(函数模板)</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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</body>
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</html>
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