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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_stringbuf&lt;CharT,Traits,Allocator&gt;::</span>underflow</h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-io.html">io</a>&lrm; | <a href="cpp-io-basic_stringbuf.html">basic stringbuf</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 />
<span class="kw1">virtual</span> int_type underflow<span class="br0">(</span><span class="br0">)</span></span></div>
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<p>&#20174;&#32531;&#20914;&#30340;&#33719;&#21462;&#21306;&#35835;&#21462;&#19979;&#20010;&#23383;&#31526;&#12290;</p>
<p>&#20855;&#20307;&#32780;&#35328;&#65306;</p>
<div class="t-li1"><span class="t-li">1)</span> &#33509;&#36755;&#20837;&#24207;&#21015;&#25317;&#26377;&#21487;&#29992;&#30340;&#35835;&#20301;&#32622;&#65288; <span class="t-c"><span class="mw-geshi cpp source-cpp">egptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">&gt;</span> gptr<span class="br0">(</span><span class="br0">)</span></span></span> &#65289;&#65292;&#21017;&#36820;&#22238; <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> &#12290;</div>
<div class="t-li1"><span class="t-li">2)</span> &#21542;&#21017;&#65292;&#33509; <span class="t-c"><span class="mw-geshi cpp source-cpp">pptr<span class="br0">(</span><span class="br0">)</span> <span class="sy1">&gt;</span> egptr<span class="br0">(</span><span class="br0">)</span></span></span> &#65288;&#20174;&#19978;&#27425;&#26356;&#25913; <span class="t-lc"><a href="cpp-io-basic_streambuf-gptr.html">egptr()</a></span> &#30340; <span class="t-lc"><a href="cpp-io-basic_stringbuf-overflow.html">overflow()</a></span> &#36215;&#25554;&#20837;&#20102;&#19968;&#20123;&#23383;&#31526;&#21040;&#27969;&#20013;&#65289;&#65292;&#21017;&#36890;&#36807;&#26356;&#25913; <span class="t-lc"><a href="cpp-io-basic_streambuf-gptr.html">egptr()</a></span> &#20026;&#31561;&#20110; <span class="t-lc"><a href="cpp-io-basic_streambuf-pptr.html">pptr()</a></span> &#65292;&#25193;&#23637;&#33719;&#21462;&#21306;&#32467;&#23614;&#20197;&#21253;&#21547;&#26368;&#36817;&#25554;&#20837;&#30340;&#23383;&#31526;&#65292;&#20877;&#36820;&#22238; <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> &#12290;</div>
<div class="t-li1"><span class="t-li">3)</span> &#21542;&#21017;&#65292;&#36820;&#22238; <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> &#12290;</div>
<p>&#32531;&#20914;&#21306;&#20013;&#20219;&#20309;&#24050;&#34987;&#21021;&#22987;&#21270;&#30340;&#23383;&#31526;&#65292;&#26080;&#20851;&#20046;&#23427;&#28304;&#33258;&#26500;&#36896;&#20989;&#25968;&#20013;&#20256;&#36882;&#30340;&#23383;&#31526;&#20018;&#36824;&#26159;&#20026; <span class="t-lc"><a href="cpp-io-basic_stringbuf-overflow.html">overflow()</a></span> &#25152;&#21518;&#38468;&#65292;&#37117;&#34987;&#35748;&#20026;&#26159;&#36755;&#20837;&#24207;&#21015;&#30340;&#19968;&#37096;&#20998;&#12290;</p>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">&#21442;&#25968;</span></h3>
<p>&#65288;&#26080;&#65289;</p>
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">&#36820;&#22238;&#20540;</span></h3>
<p>&#25104;&#21151;&#24773;&#20917;&#19979;&#20026; <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> &#65288;&#33719;&#21462;&#21306;&#20013;&#35201;&#35835;&#21462;&#30340;&#19979;&#20010;&#23383;&#31526;&#65289;&#65292;&#22833;&#36133;&#24773;&#20917;&#19979;&#20026; <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> &#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<pre class="de1"><span class="co2">#include &lt;sstream&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">struct</span> mybuf <span class="sy4">:</span> <a href="cpp-io-basic_stringbuf.html"><span class="kw1726">std::<span class="me2">stringbuf</span></span></a>
<span class="br0">{</span>
mybuf<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> new_str,
<a href="cpp-io-ios_base-openmode.html"><span class="kw1671">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">openmode</span></span></a> which <span class="sy1">=</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="sy3">|</span><a href="cpp-io-ios_base-openmode.html"><span class="kw1675">std::<span class="me2">ios_base</span><span class="sy4">::</span><span class="me2">out</span></span></a><span class="br0">)</span>
<span class="sy4">:</span> <a href="cpp-io-basic_stringbuf.html"><span class="kw1726">std::<span class="me2">stringbuf</span></span></a><span class="br0">(</span>new_str, which<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
int_type overflow<span class="br0">(</span>int_type c<span class="br0">)</span> <span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;Before overflow(): get area size is &quot;</span> <span class="sy1">&lt;&lt;</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">&lt;&lt;</span> <span class="st0">&#39; &#39;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; the put area size is &quot;</span> <span class="sy1">&lt;&lt;</span> epptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>pbase<span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
int_type rc <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">stringbuf</span><span class="sy4">::</span><span class="me2">overflow</span><span class="br0">(</span>c<span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;After overflow(): get area size is &quot;</span> <span class="sy1">&lt;&lt;</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">&lt;&lt;</span> <span class="st0">&#39; &#39;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; put area size is &quot;</span> <span class="sy1">&lt;&lt;</span> epptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>pbase<span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw1">return</span> rc<span class="sy4">;</span>
<span class="br0">}</span>
int_type underflow<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;Before underflow(): get area size is &quot;</span> <span class="sy1">&lt;&lt;</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">&lt;&lt;</span> <span class="st0">&#39; &#39;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; put area size is &quot;</span> <span class="sy1">&lt;&lt;</span> epptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>pbase<span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
int_type ch <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">stringbuf</span><span class="sy4">::</span><span class="me2">underflow</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;After underflow(): get area size is &quot;</span> <span class="sy1">&lt;&lt;</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">&lt;&lt;</span> <span class="st0">&#39; &#39;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; put area size is &quot;</span> <span class="sy1">&lt;&lt;</span> epptr<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span>pbase<span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span>ch <span class="sy1">==</span> <a href="cpp-io-c.html"><span class="kw1882">EOF</span></a><span class="br0">)</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;underflow() returns EOF<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">else</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;underflow() returns &#39;&quot;</span> <span class="sy1">&lt;&lt;</span> <span class="kw4">char</span><span class="br0">(</span>ch<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;&#39;<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">return</span> ch<span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
mybuf sbuf<span class="br0">(</span><span class="st0">&quot;123&quot;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// &#35835;&#20889;&#27969;</span>
<a href="cpp-io-basic_iostream.html"><span class="kw1731">std::<span class="me2">iostream</span></span></a> stream<span class="br0">(</span><span class="sy3">&amp;</span>sbuf<span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">int</span> n<span class="sy4">;</span>
stream <span class="sy1">&gt;&gt;</span> n<span class="sy4">;</span> <span class="co1">// &#35843;&#29992; sgetc() &#22235;&#27425;</span>
<span class="co1">// &#19977;&#27425;&#35843;&#29992;&#36820;&#22238;&#23383;&#31526; &#39;1&#39; &#12289; &#39;2&#39; &#12289; &#39;3&#39;</span>
<span class="co1">// &#31532;&#22235;&#27425;&#35843;&#29992;&#65292; gptr() == egptr() &#24182;&#35843;&#29992; underflow()</span>
<span class="co1">// underflow &#36820;&#22238; EOF</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> n <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
stream.<span class="me1">clear</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// &#28165;&#38500; eofbit</span>
stream <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;123456&quot;</span><span class="sy4">;</span> <span class="co1">// &#35843;&#29992; sputc() 6 &#27425;</span>
<span class="co1">// &#21069;&#19977;&#27425;&#35843;&#29992;&#23384;&#20648; &quot;123&quot; &#20110;&#26082;&#23384;&#32531;&#20914;&#21306;</span>
<span class="co1">// &#31532; 4 &#27425;&#35843;&#29992;&#21457;&#29616; pptr() == epptr() &#24182;&#35843;&#29992; overflow()</span>
<span class="co1">// overflow() &#20196;&#32531;&#20914;&#21306;&#29983;&#38271;&#24182;&#35774;&#32622; egptr() &#20026; 4</span>
<span class="co1">// &#31532; 5 &#21644;&#31532; 6 &#27425;&#35843;&#29992;&#23384;&#20648; &#39;5&#39; &#21644; &#39;6&#39; &#65292;&#20196; pptr() &#21069;&#36827;</span>
stream <span class="sy1">&gt;&gt;</span> n<span class="sy4">;</span> <span class="co1">// &#35843;&#29992; sgetc() 4 &#27425;</span>
<span class="co1">// &#31532; 1 &#27425;&#35843;&#29992;&#36820;&#22238; overflow() &#20351;&#20043;&#21487;&#29992;&#30340; &#39;4&#39;</span>
<span class="co1">// &#31532; 2 &#27425;&#35843;&#29992;&#26102;&#65292; egptr() == egptr() &#24182;&#35843;&#29992; underflow()</span>
<span class="co1">// underflow &#20196; egptr() &#21069;&#36827;&#21040;&#31561;&#20110; pptr() (which is 6)</span>
<span class="co1">// &#31532; 3 &#27425; sgetc() &#36820;&#22238; &#39;6&#39;</span>
<span class="co1">// &#31532; 4 &#27425; sgetc() &#21457;&#29616; gptr() == egptr() &#65292;&#35843;&#29992; underflow()</span>
<span class="co1">// underflow() &#36820;&#22238; EOF</span>
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> n <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#21487;&#33021;&#30340;&#36755;&#20986;&#65306;</p>
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<div class="text source-text">
<pre class="de1">Before underflow(): get area size is 3 put area size is 3
After underflow(): get area size is 3 put area size is 3
underflow() returns EOF
123
Before overflow(): get area size is 3 the put area size is 3
After overflow(): get area size is 4 put area size is 35
Before underflow(): get area size is 4 put area size is 35
After underflow(): get area size is 6 put area size is 35
underflow() returns &#39;5&#39;
Before underflow(): get area size is 6 put area size is 35
After underflow(): get area size is 6 put area size is 35
underflow() returns EOF
456</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
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<div class="t-dsc-member-div">
<div><a href="cpp-io-basic_streambuf-underflow.html"><span class="t-lines"><span>underflow</span></span></a></div>
<div><span class="t-lines"><span><span class="t-cmark">[&#34394;]</span></span></span></div>
</div>
</td>
<td>&#20174;&#20851;&#32852;&#36755;&#20837;&#24207;&#21015;&#35835;&#21462;&#23383;&#31526;&#21040;&#33719;&#21462;&#21306;<br />
<span class="t-mark">(<code>std::basic_streambuf&lt;CharT,Traits&gt;</code> &#30340;&#34394;&#21463;&#20445;&#25252;&#25104;&#21592;&#20989;&#25968;)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-io-basic_filebuf-underflow.html"><span class="t-lines"><span>underflow</span></span></a></div>
<div><span class="t-lines"><span><span class="t-cmark">[&#34394;]</span></span></span></div>
</div>
</td>
<td>&#20174;&#20851;&#32852;&#25991;&#20214;&#35835;&#21462;<br />
<span class="t-mark">(<code>std::basic_filebuf&lt;CharT,Traits&gt;</code> &#30340;&#34394;&#21463;&#20445;&#25252;&#25104;&#21592;&#20989;&#25968;)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-io-strstreambuf-underflow.html"><span class="t-lines"><span>underflow</span></span></a></div>
<div><span class="t-lines"><span><span class="t-cmark">[&#34394;]</span></span></span></div>
</div>
</td>
<td>&#20174;&#36755;&#20837;&#24207;&#21015;&#35835;&#21462;&#19968;&#20010;&#23383;&#31526;&#32780;&#19981;&#21069;&#36827;&#19979;&#19968;&#20301;&#32622;&#25351;&#38024;<br />
<span class="t-mark">(<code>std::strstreambuf</code> &#30340;&#34394;&#21463;&#20445;&#25252;&#25104;&#21592;&#20989;&#25968;)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-io-basic_streambuf-sgetc.html"><span class="t-lines"><span>sgetc</span></span></a></div>
</div>
</td>
<td>&#20174;&#36755;&#20837;&#24207;&#21015;&#35835;&#21462;&#19968;&#20010;&#23383;&#31526;&#65292;&#32780;&#19981;&#20196;&#24207;&#21015;&#21069;&#36827;<br />
<span class="t-mark">(<code>std::basic_streambuf&lt;CharT,Traits&gt;</code> &#30340;&#20844;&#24320;&#25104;&#21592;&#20989;&#25968;)</span></td>
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