167 lines
13 KiB
HTML
167 lines
13 KiB
HTML
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<title>std::basic_istream<CharT,Traits>::read</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_basic_istream_read 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>read</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-basic_istream.html">basic istream</a></span></div>
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<div><span class="mw-geshi cpp source-cpp">basic_istream<span class="sy3">&</span> read<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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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>从流释出字符。</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，释出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。释出并存储字符，直至出现任何下列条件：</p>
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<ul>
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<li>释出并存储了 <code>count</code> 个字符</li>
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</ul>
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<ul>
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<li>输入序列上的文件尾条件（该情况下调用 <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> ）。成功释出的字符数能用 <a href="cpp-io-basic_istream-gcount.html">gcount()</a> 查询。</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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<table class="t-par-begin">
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<tr class="t-par">
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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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</tr>
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<tr class="t-par">
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<td>count</td>
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<td>-</td>
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<td>要读取的字符数</td>
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</tr>
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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><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span></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>使用非转换的本地环境时（默认本地环境为非转换），此函数在 <span class="t-lc"><a href="cpp-io-basic_ifstream.html">std::basic_ifstream</a></span> 中的覆写者可以为零复制的大块 I/O 优化（通过覆写 <span class="t-lc"><a href="cpp-io-basic_streambuf-sgetn.html">std::streambuf::xsgetn</a></span> ）。</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="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 <fstream></span>
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<span class="co2">#include <sstream></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <cstdint></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="co1">// read() 常用于二进制 I/O</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> bin <span class="sy1">=</span> <span class="br0">{</span><span class="st0">'<span class="es2">\x12</span>'</span>, <span class="st0">'<span class="es2">\x12</span>'</span>, <span class="st0">'<span class="es2">\x12</span>'</span>, <span class="st0">'<span class="es2">\x12</span>'</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> raw<span class="br0">(</span>bin<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-types-integer.html"><span class="kw122">std::<span class="me2">uint32_t</span></span></a> n<span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>raw.<span class="me1">read</span><span class="br0">(</span><span class="kw1">reinterpret_cast</span><span class="sy1"><</span><span class="kw4">char</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>n<span class="br0">)</span>, sizeof n<span class="br0">)</span><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-io-manip-hex.html"><span class="kw1782">std::<span class="me2">hex</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-showbase.html"><span class="kw1766">std::<span class="me2">showbase</span></span></a> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// 为下个片段准备文件</span>
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<a href="cpp-io-basic_ofstream.html"><span class="kw1733">std::<span class="me2">ofstream</span></span></a><span class="br0">(</span><span class="st0">"test.txt"</span>, std<span class="sy4">::</span><span class="me2">ios</span><span class="sy4">::</span><span class="me2">binary</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"abcd1<span class="es1">\n</span>abcd2<span class="es1">\n</span>abcd3"</span><span class="sy4">;</span>
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<span class="co1">// 读取整个文件到 string</span>
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<span class="kw1">if</span><span class="br0">(</span><a href="cpp-io-basic_ifstream.html"><span class="kw1732">std::<span class="me2">ifstream</span></span></a> is<span class="br0">{</span><span class="st0">"test.txt"</span>, std<span class="sy4">::</span><span class="me2">ios</span><span class="sy4">::</span><span class="me2">binary</span> <span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">ios</span><span class="sy4">::</span><span class="me2">ate</span><span class="br0">}</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">auto</span> size <span class="sy1">=</span> is.<span class="me1">tellg</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str<span class="br0">(</span>size, <span class="st0">'<span class="es5">\0</span>'</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 构造 string 为流大小</span>
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is.<span class="me1">seekg</span><span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>is.<span class="me1">read</span><span class="br0">(</span><span class="sy3">&</span>str<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>, size<span class="br0">)</span><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> str <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>
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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">0x12121212
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abcd1
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abcd2
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abcd3</pre></div>
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</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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-basic_ostream-write.html"><span class="t-lines"><span>write</span></span></a></div>
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</div>
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</td>
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<td>插入字符块<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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</tr>
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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-readsome.html"><span class="t-lines"><span>readsome</span></span></a></div>
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</div>
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</td>
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<td>读并取走已经可用的字符块<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-get.html"><span class="t-lines"><span>get</span></span></a></div>
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</div>
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</td>
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<td>从流中读并取走（移除类似指针向下一个元素移动）一个字符<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-getline.html"><span class="t-lines"><span>getline</span></span></a></div>
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</div>
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</td>
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<td>一直读并取走字符，直至找到给定字符<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-c-fread.html"><span class="t-lines"><span>fread</span></span></a></div>
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</div>
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</td>
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<td>从文件读取<br />
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<span class="t-mark">(函数)</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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