115 lines
10 KiB
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115 lines
10 KiB
HTML
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<title>std::basic_istream<CharT,Traits>::readsome</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_basic_istream_readsome 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::basic_istream<CharT,Traits>::</span>readsome</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"><a href="cpp-io-streamsize.html"><span class="kw1804">std::<span class="me2">streamsize</span></span></a> readsome<span class="br0">(</span> char_type<span class="sy2">*</span> s, <a href="cpp-io-streamsize.html"><span class="kw1804">std::<span class="me2">streamsize</span></span></a> count <span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>从输入流释出至多 <code>count</code> 个立即可用的字符。存储释出的字符于 <code>s</code> 所指向的字符数组。</p>
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<p>表现为<a href="cpp-named_req-UnformattedInputFunction.html"><span style="font-style:italic">无格式输入函数</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(UnformattedInputFunction)</span></a> 。构造并检查 sentry 对象后，</p>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">rdbuf<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>in_avail<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="sy2">-</span><span class="nu0">1</span></span></span> ，则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">setstate<span class="br0">(</span>eofbit<span class="br0">)</span></span></span> 而不释出字符。</li>
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</ul>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">rdbuf<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>in_avail<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span></span></span> ，则不释出字符。</li>
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</ul>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">rdbuf<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>in_avail<span class="br0">(</span><span class="br0">)</span> <span class="sy1">></span> <span class="nu0">0</span></span></span> ，则释出 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-algorithm-min.html"><span class="kw1642">std::<span class="me2">min</span></span></a><span class="br0">(</span>rdbuf<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>in_avail<span class="br0">(</span><span class="br0">)</span>, count<span class="br0">)</span></span></span> 个字符，并存储它们到首元素为 <code>s</code> 所指向的字符数组的相继位置。</li>
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</ul>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>s</td>
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<td>-</td>
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<td>指向要存储字符到的字符数组的指针</td>
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<td>count</td>
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<td>-</td>
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<td>要读取的最大字符数</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>实际释出的字符数。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<div class="t-template-editlink" style=""></div>
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若出现错误（错误状态标志不是 <span class="t-lc"><a href="cpp-io-ios_base-iostate.html">goodbit</a></span> ）并且设置了 <span class="t-lc"><a href="cpp-io-basic_ios-exceptions.html">exceptions()</a></span> 为对该状态抛出则为 <span class="t-lc"><a href="cpp-io-ios_base-failure.html">failure</a></span> 。
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<p>若内部操作抛出异常，则捕获它并设置 <span class="t-lc"><a href="cpp-io-ios_base-iostate.html">badbit</a></span> 。若对 <code>badbit</code> 设置了 <span class="t-lc"><a href="cpp-io-basic_ios-exceptions.html">exceptions()</a></span> ，则重抛该异常。</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>此函数的行为是高度实现限定的。例如，以 <span class="t-lc"><a href="cpp-io-basic_ifstream.html">std::ifstream</a></span> 使用时，某些库实现在文件打开时立即以数据填充底层 filebuf （而这种实现上 readsome() 读取的数据潜在地，但不必为整个文件），而其他实现仅在请求实际输入操作时从文件读取（而文件打开后立即作出的 readsome() 决不释出任何字符）。类似地，调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-cin.html"><span class="kw1756">std::<span class="me2">cin</span></span></a>.<span class="me1">readsome</span><span class="br0">(</span><span class="br0">)</span></span></span> 可能返回所有悬置的未处理控制台输入，或可能始终返回零并且不释出字符。</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="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="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">char</span> c<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-io-basic_istringstream.html"><span class="kw1735">std::<span class="me2">istringstream</span></span></a> input<span class="br0">(</span><span class="st0">"This is sample text."</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// std::stringbuf 令个缓冲可用于无阻塞读取</span>
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input.<span class="me1">readsome</span><span class="br0">(</span>c, <span class="nu0">5</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 读取 'This ' 并存储于 c[0] .. c[4]</span>
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input.<span class="me1">readsome</span><span class="br0">(</span>c, <span class="nu0">9</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 读取 'is sample' 并存储于 c[0] .. c[8]</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> c<span class="sy4">;</span>
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<span class="br0">}</span></pre></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">is sample</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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<div class="t-dsc-member-div">
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<div><a href="cpp-io-basic_istream-read.html"><span class="t-lines"><span>read</span></span></a></div>
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</div>
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</td>
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<td>读并取走一块字符<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-basic_streambuf-in_avail.html"><span class="t-lines"><span>in_avail</span></span></a></div>
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</div>
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</td>
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<td>获得获取区中立即可用的字符数<br />
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<span class="t-mark">(<code>std::basic_streambuf<CharT,Traits></code> 的公开成员函数)</span></td>
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