116 lines
9.8 KiB
HTML
116 lines
9.8 KiB
HTML
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<title>std::basic_filebuf<CharT,Traits>::pbackfail</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_basic_filebuf_pbackfail 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_filebuf<CharT,Traits>::</span>pbackfail</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_filebuf.html">basic filebuf</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">protected</span><span class="sy4">:</span><br />
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<span class="kw1">virtual</span> int_type pbackfail<span class="br0">(</span> int_type c <span class="sy1">=</span> Traits<span class="sy4">::</span><span class="me2">eof</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">)</span></span></div>
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<p>此受保护虚函数为公开函数 <a href="cpp-io-basic_streambuf-sungetc.html"><tt>basic_streambuf::sungetc</tt></a> 和 <a href="cpp-io-basic_streambuf-sputbackc.html"><tt>basic_streambuf::sputbackc</tt></a> 所调用（继而为 <a href="cpp-io-basic_istream-unget.html"><tt>basic_istream::unget</tt></a> 和 <a href="cpp-io-basic_istream-putback.html"><tt>basic_istream::putback</tt></a> 所调用）。</p>
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<div class="t-li1"><span class="t-li">1)</span> 调用方请求将获取区后备一个字符（以无参数调用 <code>pbackfail()</code> ），该情况下，此函数于前一字节位置开始重读文件，并自减 <a href="cpp-io-basic_streambuf-gptr.html"><tt>basic_streambuf::gptr()</tt></a> ，例如通过调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">gbump<span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 调用方试图回放异于之前取得的字符（以需要放回的字符调用 <code>pbackfail()</code> ），该情况下</div>
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<div class="t-li2"><span class="t-li">a)</span> 首先检查是否有回放位置，若无则通过于前一字节位置开始重读文件，后备获取区。</div>
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<div class="t-li2"><span class="t-li">a)</span> 然后检查什么字符在回放位置。若其处保有的字符已等于 <code>c</code> ，以 <span class="t-c"><span class="mw-geshi cpp source-cpp">Traits<span class="sy4">::</span><span class="me2">eq</span><span class="br0">(</span>to_char_type<span class="br0">(</span>c<span class="br0">)</span>, gptr<span class="br0">(</span><span class="br0">)</span><span class="br0">[</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">]</span><span class="br0">)</span></span></span> 确定，则简单地自减 <a href="cpp-io-basic_streambuf-gptr.html"><tt>basic_streambuf::gptr()</tt></a> 。</div>
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<div class="t-li2"><span class="t-li">b)</span> 否则，若允许缓冲修改其自身的获取区，则自减 <a href="cpp-io-basic_streambuf-gptr.html"><tt>basic_streambuf::gptr()</tt></a> 并写入 <code>c</code> 到调整后的 gptr() 所指向的位置。</div>
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<p>此函数决不修改文件，仅修改内存中缓冲区的获取区。</p>
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<p>若文件未打开（ <span class="t-c"><span class="mw-geshi cpp source-cpp">is_open<span class="br0">(</span><span class="br0">)</span><span class="sy1">==</span><span class="kw2">false</span></span></span> ），则此函数立即返回 <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> 。</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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<td>c</td>
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<td>-</td>
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<td>要放回的字符，或为指示请求后备缓冲区的 Traits::eof()</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>成功时为 <span class="t-c"><span class="mw-geshi cpp source-cpp">c</span></span> ，除非 <code>c</code> 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> ，该情况下返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">Traits<span class="sy4">::</span><span class="me2">not_eof</span><span class="br0">(</span>c<span class="br0">)</span></span></span> 。</p>
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<p>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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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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-pbackfail.html"><span class="t-lines"><span>pbackfail</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[虚]</span></span></span></div>
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</div>
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</td>
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<td>将字符放回输入序列，可能修改输入序列<br />
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<span class="t-mark">(<code>std::basic_streambuf<CharT,Traits></code> 的虚受保护成员函数)</span></td>
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<div><a href="cpp-io-basic_streambuf-sungetc.html"><span class="t-lines"><span>sungetc</span></span></a></div>
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</div>
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<td>回移一位输出序列中的下一位置指针<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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<div><a href="cpp-io-basic_streambuf-sputbackc.html"><span class="t-lines"><span>sputbackc</span></span></a></div>
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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">(<code>std::basic_streambuf<CharT,Traits></code> 的公开成员函数)</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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</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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<div><a href="cpp-io-basic_istream-putback.html"><span class="t-lines"><span>putback</span></span></a></div>
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</td>
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<td>往输入流中退回一个字符<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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