99 lines
8.9 KiB
HTML
99 lines
8.9 KiB
HTML
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<title>std::basic_streambuf<CharT,Traits>::sungetc</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_streambuf<CharT,Traits>::</span>sungetc</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_streambuf.html">basic streambuf</a></span></div>
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<div><span class="mw-geshi cpp source-cpp">int_type sungetc<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>若回放位置在获取区中可用（ <span class="t-c"><span class="mw-geshi cpp source-cpp">gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">></span> eback<span class="br0">(</span><span class="br0">)</span></span></span> ），则减少下一位置指针（ <span class="t-lc"><a href="cpp-io-basic_streambuf-gptr.html">gptr()</a></span> ）并返回它现在指向的字符。</p>
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<p>若回放位置不可用，则调用 <span class="t-lc"><a href="cpp-io-basic_streambuf-pbackfail.html">pbackfail()</a></span> 以在可能的情况下后备输入序列。</p>
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<p>I/O 流函数 <a href="cpp-io-basic_istream-unget.html"><tt>basic_istream::unget</tt></a> 以此函数实现。</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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<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>若回放位置可用，则返回下一位置指针现在指向的字符，以 <span class="t-c"><span class="mw-geshi cpp source-cpp">Traits<span class="sy4">::</span><span class="me2">to_int_type</span><span class="br0">(</span><span class="sy2">*</span>gptr<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span></span></span> 转换为 <code>int_type</code> 。来自此 streambuf 的下个单字符输入将返回此字符。</p>
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<p>若回放位置不可用，则返回 <span class="t-lc"><a href="cpp-io-basic_streambuf-pbackfail.html">pbackfail()</a></span> 所返回者，在失败时为 <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> 。</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 <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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<a href="cpp-io-basic_stringstream.html"><span class="kw1737">std::<span class="me2">stringstream</span></span></a> s<span class="br0">(</span><span class="st0">"abcdef"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// gptr() 指向 'a'</span>
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<span class="kw4">char</span> c1 <span class="sy1">=</span> s.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// c = 'a', gptr() 现在指向 'b' </span>
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<span class="kw4">char</span> c2 <span class="sy1">=</span> s.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>sungetc<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 同 s.unget() ： gptr() 又指向 'a'</span>
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<span class="kw4">char</span> c3 <span class="sy1">=</span> s.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// c3 = 'a' ， gptr() 现在指向 'b'</span>
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<span class="kw4">char</span> c4 <span class="sy1">=</span> s.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// c4 = 'b' ， gptr() 现在指向 'c'</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> c1 <span class="sy1"><<</span> c2 <span class="sy1"><<</span> c3 <span class="sy1"><<</span> c4 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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s.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>sungetc<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 回到 'b'</span>
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s.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>sungetc<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 回到 'a'</span>
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<span class="kw4">int</span> eof <span class="sy1">=</span> s.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>sungetc<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 无内容可反获取： pbackfail() 失败</span>
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<span class="kw1">if</span> <span class="br0">(</span>eof <span class="sy1">==</span> <a href="cpp-io-c.html"><span class="kw1882">EOF</span></a><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> <span class="st0">"Nothing to unget after 'a'<span class="es1">\n</span>"</span><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">aaab
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Nothing to unget after 'a'</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><a href="cpp-io-basic_streambuf-sputbackc.html"><span class="t-lines"><span>sputbackc</span></span></a></div>
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</td>
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<td>在输入序列中放回一个字符<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div><a href="cpp-io-basic_istream-unget.html"><span class="t-lines"><span>unget</span></span></a></div>
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<td>撤销流中刚取走（移除，类似指针向后退回一个位置）的字符<br />
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<span class="t-mark">(<code>std::basic_istream<CharT,Traits></code> 的公开成员函数)</span></td>
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