150 lines
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150 lines
12 KiB
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<title>std::basic_filebuf<CharT,Traits>::underflow</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_filebuf<CharT,Traits>::</span>underflow</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_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 underflow<span class="br0">(</span><span class="br0">)</span></span></div>
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<p>读取更多数据到输入区中。</p>
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<p>表现类似基类 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">basic_streambuf</span><span class="sy4">::</span><span class="me2">underflow</span></span></span> ，除了要从关联字符序列（文件）读取数据到获取区中。首先从文件读取字符到临时缓冲区（分配所需大小），然后用感染的 locale 的 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">codecvt</span><span class="sy4">::</span><span class="me2">in</span></span></span> 转换外部（典型为多字节）表示为之后用于填充获取区的内部形式。若该 locale 的 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">codecvt</span><span class="sy4">::</span><span class="me2">always_noconv</span></span></span> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 则可以跳过转换。</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>成功情况下为 <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> （待处理序列的首字符），失败情况下为 <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 <fstream></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> mybuf <span class="sy4">:</span> <a href="cpp-io-basic_filebuf.html"><span class="kw1725">std::<span class="me2">filebuf</span></span></a>
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<span class="br0">{</span>
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<span class="kw4">int</span> underflow<span class="br0">(</span><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> <span class="st0">"Before underflow(): size of the get area is "</span>
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<span class="sy1"><<</span> egptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>eback<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" with "</span>
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<span class="sy1"><<</span> egptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" read positions available<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw4">int</span> rc <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">filebuf</span><span class="sy4">::</span><span class="me2">underflow</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</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">"underflow() returns "</span> <span class="sy1"><<</span> rc <span class="sy1"><<</span> <span class="st0">".<span class="es1">\n</span>After the call, "</span>
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<span class="sy1"><<</span> <span class="st0">"size of the get area is "</span>
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<span class="sy1"><<</span> egptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>eback<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" with "</span>
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<span class="sy1"><<</span> egptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" read positions available<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> rc<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</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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mybuf buf<span class="sy4">;</span>
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buf.<span class="me1">open</span><span class="br0">(</span><span class="st0">"test.txt"</span>, <a href="cpp-io-ios_base-openmode.html"><span class="kw1674">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">in</span></span></a><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-basic_istream.html"><span class="kw1727">std::<span class="me2">istream</span></span></a> stream<span class="br0">(</span><span class="sy3">&</span>buf<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">while</span><span class="br0">(</span>stream.<span class="me1">get</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<p>可能的输出：</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">Before underflow(): size of the get area is 0 with 0 read positions available
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underflow() returns 73.
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After the call, size of the get area is 110 with 110 read positions available
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Before underflow(): size of the get area is 110 with 0 read positions available
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underflow() returns -1.
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After the call, size of the get area is 0 with 0 read positions available</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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<table class="t-dsc-begin">
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-basic_streambuf-underflow.html"><span class="t-lines"><span>underflow</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>从关联输入序列读取字符到获取区<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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</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_stringbuf-underflow.html"><span class="t-lines"><span>underflow</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>返回输入序列中可用的下一字符<br />
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<span class="t-mark">(<code>std::basic_stringbuf<CharT,Traits,Allocator></code> 的虚受保护成员函数)</span></td>
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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-strstreambuf-underflow.html"><span class="t-lines"><span>underflow</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>从输入序列读取一个字符而不前进下一位置指针<br />
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<span class="t-mark">(<code>std::strstreambuf</code> 的虚受保护成员函数)</span></td>
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</tr>
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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_filebuf-uflow.html"><span class="t-lines"><span>uflow</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>从关联文件读取，并令获取区的下一位置指针前进<br />
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<span class="t-mark">(虚受保护成员函数)</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_filebuf-overflow.html"><span class="t-lines"><span>overflow</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>从放置区写字符到关联的文件<br />
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<span class="t-mark">(虚受保护成员函数)</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_streambuf-sgetc.html"><span class="t-lines"><span>sgetc</span></span></a></div>
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</div>
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</td>
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<td>从输入序列读取一个字符，而不令序列前进<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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