153 lines
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153 lines
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<title>std::basic_streambuf<CharT,Traits>::underflow</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>underflow</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_streambuf.html">basic streambuf</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">virtual</span> int_type underflow<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>通过更新指向输入区的指针（若需要），和从输入序列读取更多数据（若可用），确保输入区中至少一个字符可用。成功时返回该字符的值（以 <code>int_type</code> 转换为 <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>c<span class="br0">)</span></span></span> ），失败时返回 <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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<p>函数可以更新 <code>gptr</code> 、 <code>egptr</code> 和 <code>eback</code> 指针，以定义新加载的数据位置（若存在）。失败时，函数确保 <span class="t-c"><span class="mw-geshi cpp source-cpp">gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> nullptr</span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> egptr</span></span> 。</p>
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<p>函数的基类版本不做任何事。导出类可覆写此函数以允许在耗尽情况下更新获取区。</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>成功时为调用后<i>获取指针</i>所指向的字符的值。否则为 <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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<p>函数的基类版本返回 <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=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p><span class="t-lc"><a href="cpp-io-basic_streambuf.html">std::streambuf</a></span> 的公开函数仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp">gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> nullptr</span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">gptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">>=</span> egptr<span class="br0">(</span><span class="br0">)</span></span></span> 才调用此函数。</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-template-editlink" style=""></div>
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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="kw1">class</span> null_filter_buf <span class="sy4">:</span> <span class="kw1">public</span> <a href="cpp-io-basic_streambuf.html"><span class="kw1724">std::<span class="me2">streambuf</span></span></a> <span class="br0">{</span>
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<a href="cpp-io-basic_streambuf.html"><span class="kw1724">std::<span class="me2">streambuf</span></span></a><span class="sy2">*</span> src<span class="sy4">;</span>
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<span class="kw4">char</span> ch<span class="sy4">;</span> <span class="co1">// 单字节缓冲区</span>
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<span class="kw1">protected</span><span class="sy4">:</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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traits_type<span class="sy4">::</span><span class="me2">int_type</span> i<span class="sy4">;</span>
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<span class="kw1">while</span> <span class="br0">(</span><span class="br0">(</span>i <span class="sy1">=</span> src<span class="sy2">-</span><span class="sy1">></span>sbumpc<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1">==</span> <span class="st0">'<span class="es5">\0</span>'</span><span class="br0">)</span> <span class="sy4">;</span> <span class="co1">// 跳过零</span>
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<span class="kw1">if</span> <span class="br0">(</span>traits_type<span class="sy4">::</span><span class="me2">not_eof</span><span class="br0">(</span>i<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
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ch <span class="sy1">=</span> traits_type<span class="sy4">::</span><span class="me2">to_char_type</span><span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span>
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setg<span class="br0">(</span><span class="sy3">&</span>ch, <span class="sy3">&</span>ch, <span class="sy3">&</span>ch<span class="sy2">+</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 使得一个读取位置可用</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> i<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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null_filter_buf<span class="br0">(</span><a href="cpp-io-basic_streambuf.html"><span class="kw1724">std::<span class="me2">streambuf</span></span></a><span class="sy2">*</span> buf<span class="br0">)</span> <span class="sy4">:</span> src<span class="br0">(</span>buf<span class="br0">)</span> <span class="br0">{</span>
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setg<span class="br0">(</span><span class="sy3">&</span>ch, <span class="sy3">&</span>ch<span class="sy2">+</span><span class="nu0">1</span>, <span class="sy3">&</span>ch<span class="sy2">+</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 缓冲区初始为满</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">void</span> filtered_read<span class="br0">(</span><a href="cpp-io-basic_istream.html"><span class="kw1727">std::<span class="me2">istream</span></span></a><span class="sy3">&</span> in<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-basic_streambuf.html"><span class="kw1724">std::<span class="me2">streambuf</span></span></a><span class="sy2">*</span> orig <span class="sy1">=</span> in.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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null_filter_buf buf<span class="br0">(</span>orig<span class="br0">)</span><span class="sy4">;</span>
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in.<span class="me1">rdbuf</span><span class="br0">(</span><span class="sy3">&</span>buf<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">char</span> c<span class="sy4">;</span> in.<span class="me1">get</span><span class="br0">(</span>c<span class="br0">)</span><span class="sy4">;</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> c<span class="sy4">;</span>
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in.<span class="me1">rdbuf</span><span class="br0">(</span>orig<span class="br0">)</span><span class="sy4">;</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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<span class="kw4">char</span> a<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">"This i<span class="es5">\0</span>s <span class="es5">\0</span>an e<span class="es5">\0</span><span class="es5">\0</span><span class="es5">\0</span>xample"</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> in<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">(</span><a href="cpp-iterator-begin.html"><span class="kw676">std::<span class="me2">begin</span></span></a><span class="br0">(</span>a<span class="br0">)</span>, <a href="cpp-iterator-end.html"><span class="kw680">std::<span class="me2">end</span></span></a><span class="br0">(</span>a<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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filtered_read<span class="br0">(</span>in<span class="br0">)</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">This is an example</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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<div class="t-dsc-member-div">
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<div><a href="cpp-io-basic_streambuf-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>从输入序列读取字符到获取区，并令下一位置指针前进<br />
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<span class="t-mark">(虚受保护成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-basic_streambuf-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>从放置区写入字符到关联的输出序列<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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<div class="t-dsc-member-div">
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<div><a href="cpp-io-basic_filebuf-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>从关联文件读取<br />
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<span class="t-mark">(<code>std::basic_filebuf<CharT,Traits></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-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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