131 lines
12 KiB
HTML
131 lines
12 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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<title>std::ungetc</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_c_ungetc 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>ungetc</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-c.html">c</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-cstdio.html"><cstdio></a></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> ungetc<span class="br0">(</span> <span class="kw4">int</span> ch, <a href="cpp-io-c.html"><span class="kw1876">std::<span class="me2">FILE</span></span></a> <span class="sy2">*</span>stream <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<tr class="t-dcl-sep">
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<td></td>
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<p>若 <code>ch</code> 不等于 <span class="t-lc"><a href="cpp-io-c.html">EOF</a></span> ，则推入字符 <code>ch</code> （转译为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ）到与流 <code>stream</code> 关联的输入缓冲区，方式满足从 <code>stream</code> 的后继读取操作将取得该字符。不修改与流关联的外部设备。</p>
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<p>流重寻位操作 <span class="t-lc"><a href="cpp-io-c-fseek.html">std::fseek</a></span> 、 <span class="t-lc"><a href="cpp-io-c-fsetpos.html">std::fsetpos</a></span> 和 <span class="t-lc"><a href="cpp-io-c-rewind.html">std::rewind</a></span> 弃去 <code>ungetc</code> 的效果。</p>
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<p>若调用 <code>ungetc</code> 多于一次，而无中间读取或重寻位，则可能失败（换言之，保证大小为 1 的回放缓冲区，但任何更大的缓冲区是实现定义的）。若成功进行多次 <code>ungetc</code> ，则读取操作以 <code>ungetc</code> 的逆序取得回放的字符。</p>
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<p>若 <code>ch</code> 等于 <span class="t-lc"><a href="cpp-io-c.html">EOF</a></span> ，则操作失败而不影响流。</p>
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<p>对 <code>ungetc</code> 的成功调用清除文件尾状态标志 <span class="t-lc"><a href="cpp-io-c-feof.html">std::feof</a></span> 。</p>
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<p>在二进制流上对 <code>ungetc</code> 的成功调用将流位置指示器减少一（若流位置指示器为零，则行为不确定）。</p>
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<p>在文本流上对 <code>ungetc</code> 的成功调用以未指定方式修改流位置指示器，但保证在以读取操作取得所有回放字符后，流位置指示器等于其在 <code>ungetc</code> 之前的值。</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>ch</td>
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<td>-</td>
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<td>要推入输入流缓冲区的字符</td>
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</tr>
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<tr class="t-par">
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<td>stream</td>
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<td>-</td>
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<td>要回放字符到的文件流</td>
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</tr>
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</table>
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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>成功时返回 <code>ch</code> 。</p>
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<p>失败时返回 <span class="t-lc"><a href="cpp-io-c.html">EOF</a></span> ，而给定的流保持不变。</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>实践中，回放缓冲区的大小会在 4k （ Linux 、 MacOS ）和 4 （ Solaris ）或保证的最小值 1 （ HPUX 、 AIX ）间变化。</p>
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<p>若回放的字符等于存在于外部字符序列中该位置的字符，则回放缓冲区的表观大小可以更大（实现可以简单地自减读取的文件位置指示器，并避免维护回放缓冲区）。</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>以其原目的：实现 scanf 演示 ungetc 的使用</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 <cctype></span>
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<span class="co2">#include <cstdio></span>
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<span class="kw4">void</span> demo_scanf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> fmt, <a href="cpp-io-c.html"><span class="kw1876">std::<span class="me2">FILE</span></span></a><span class="sy2">*</span> s<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">if</span><span class="br0">(</span><span class="sy2">*</span>fmt <span class="sy1">==</span> <span class="st0">'%'</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">switch</span><span class="br0">(</span><span class="sy2">*++</span>fmt<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">case</span> <span class="st0">'u'</span><span class="sy4">:</span> <span class="br0">{</span>
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<span class="kw4">int</span> c<span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">isspace</span><span class="br0">(</span>c<span class="sy1">=</span><a href="cpp-io-c-fgetc.html"><span class="kw1822">std::<span class="me2">getc</span></span></a><span class="br0">(</span>s<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// 跳过前导空白</span>
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<span class="kw4">unsigned</span> <span class="kw4">int</span> num <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">isdigit</span><span class="br0">(</span>c<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
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num <span class="sy1">=</span> num<span class="sy2">*</span><span class="nu0">10</span> <span class="sy2">+</span> c<span class="sy2">-</span><span class="st0">'0'</span><span class="sy4">;</span>
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c <span class="sy1">=</span> <a href="cpp-io-c-fgetc.html"><span class="kw1822">std::<span class="me2">getc</span></span></a><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="cpp-io-c-fprintf.html"><span class="kw1844">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"%%u scanned %u<span class="es1">\n</span>"</span>, num<span class="br0">)</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ungetc</span><span class="br0">(</span>c, s<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 重处理非数位</span>
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<span class="br0">}</span>
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<span class="kw1">case</span> <span class="st0">'c'</span><span class="sy4">:</span> <span class="br0">{</span>
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<span class="kw4">int</span> c <span class="sy1">=</span> <a href="cpp-io-c-fgetc.html"><span class="kw1822">std::<span class="me2">getc</span></span></a><span class="br0">(</span>s<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-fprintf.html"><span class="kw1844">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"%%c scanned '%c'<span class="es1">\n</span>"</span>, c<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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<span class="br0">}</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-c.html"><span class="kw1876">std::<span class="me2">FILE</span></span></a><span class="sy2">*</span> f <span class="sy1">=</span> <a href="cpp-io-c-fopen.html"><span class="kw1813">std::<span class="me2">fopen</span></span></a><span class="br0">(</span><span class="st0">"input.txt"</span>, <span class="st0">"w+"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-fputs.html"><span class="kw1826">std::<span class="me2">fputs</span></span></a><span class="br0">(</span><span class="st0">"123x"</span>, f<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-rewind.html"><span class="kw1868">std::<span class="me2">rewind</span></span></a><span class="br0">(</span>f<span class="br0">)</span><span class="sy4">;</span>
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demo_scanf<span class="br0">(</span><span class="st0">"%u%c"</span>, f<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-c-fclose.html"><span class="kw1816">std::<span class="me2">fclose</span></span></a><span class="br0">(</span>f<span class="br0">)</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">%u scanned 123
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%c scanned 'x'</pre></div>
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</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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<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-fgetc.html"><span class="t-lines"><span>fgetc</span><span>getc</span></span></a></div>
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</div>
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</td>
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<td>从文件流获取字符<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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</div>
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</div>
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</div>
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</body>
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