150 lines
16 KiB
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150 lines
16 KiB
HTML
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<title>std::basic_stringbuf<CharT,Traits,Allocator>::overflow</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_stringbuf<CharT,Traits,Allocator>::</span>overflow</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_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 />
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<span class="kw1">virtual</span> int_type overflow <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 class="sy4">;</span></span></div>
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<td></td>
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<p>后附字符 <code>c</code> 到输出字符序列。</p>
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<p>若 <code>c</code> 为文件尾指示器（ <span class="t-c"><span class="mw-geshi cpp source-cpp">traits<span class="sy4">::</span><span class="me2">eq_int_type</span><span class="br0">(</span>c,traits<span class="sy4">::</span><span class="me2">eof</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1">==</span> <span class="kw2">true</span></span></span> ），则不后附字符。函数不做任何操作并返回异于 <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> 的未指定值。</p>
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<p>否则，若输出序列拥有可用写位置，或若此函数成功令一个写位置可用，则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">sputc<span class="br0">(</span>c<span class="br0">)</span></span></span> 并返回 <code>c</code> 。</p>
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<p>若 stringbuf 为输出打开（ <span class="t-c"><span class="mw-geshi cpp source-cpp">mode <span class="sy3">&</span> ios_base<span class="sy4">::</span><span class="me2">out</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span></span></span> ），则此函数能令写位置可用：此情况下，它重分配（或在最初分配）足够大的缓冲区，以保有整个当前缓冲区加上至少一个字符。若 stringbuf 亦为输入打开（ <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>mode <span class="sy3">&</span> ios_base<span class="sy4">::</span><span class="me2">in</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span></span></span> ），则 <code>overflow</code> 亦会通过移动 <span class="t-c"><span class="mw-geshi cpp source-cpp">egptr<span class="br0">(</span><span class="br0">)</span></span></span> 到指向恰好越过新放置区的位置，来增加获取区的大小。</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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<table class="t-par-begin">
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<td>c</td>
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<td>-</td>
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<td>要存储于放置区的字符</td>
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</tr>
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</table>
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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>若失败则为指示失败的 <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> 。若成功后附字符 <code>c</code> 则为 <code>c</code> 。或若以 <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> 为参数调用，则为某异于 <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> 的值。</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>此函数异于典型的 <code>overflow()</code> ，后者移动缓冲区的内容到关联字符序列，因为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-io-basic_stringbuf.html"><span class="kw1668">std::<span class="me2">basic_stringbuf</span></span></a></span></span> 的缓冲区和关联序列是同一序列。</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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<p>在用于执行此示例的实现中， overflow() 过分配放置区为 512 字节：调用 str() 会只返回四个初始字节，但剩下的 508 次对 sputc() 的调用不会要求对 overflow() 的新调用</p>
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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 <sstream></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_stringbuf.html"><span class="kw1726">std::<span class="me2">stringbuf</span></span></a>
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<span class="br0">{</span>
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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">&</span> new_str,
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<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>
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<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>
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int_type overflow<span class="br0">(</span>int_type c <span class="sy1">=</span> <a href="cpp-io-c.html"><span class="kw1882">EOF</span></a><span class="br0">)</span> override
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<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">"stringbuf::overflow('"</span> <span class="sy1"><<</span> <span class="kw4">char</span><span class="br0">(</span>c<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"') called<span class="es1">\n</span>"</span>
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<span class="sy1"><<</span> <span class="st0">"Before: size of get area: "</span> <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">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">" size of put area: "</span> <span class="sy1"><<</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"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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int_type ret <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>
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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">"After : size of get area: "</span> <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">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">" size of put area: "</span> <span class="sy1"><<</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"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw1">return</span> ret<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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"read-write stream:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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mybuf sbuf<span class="br0">(</span><span class="st0">" "</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 只读流</span>
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<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">&</span>sbuf<span class="br0">)</span><span class="sy4">;</span>
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stream <span class="sy1"><<</span> <span class="nu0">1234</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> sbuf.<span class="me1">str</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</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">"<span class="es1">\n</span>read-only stream:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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mybuf ro_buf<span class="br0">(</span><span class="st0">" "</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> <span class="co1">// 只读流</span>
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<a href="cpp-io-basic_iostream.html"><span class="kw1731">std::<span class="me2">iostream</span></span></a> ro_stream<span class="br0">(</span><span class="sy3">&</span>ro_buf<span class="br0">)</span><span class="sy4">;</span>
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ro_stream <span class="sy1"><<</span> <span class="nu0">1234</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">"<span class="es1">\n</span>write-only stream:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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mybuf wr_buf<span class="br0">(</span><span class="st0">" "</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> <span class="co1">// 只写流</span>
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<a href="cpp-io-basic_iostream.html"><span class="kw1731">std::<span class="me2">iostream</span></span></a> wr_stream<span class="br0">(</span><span class="sy3">&</span>wr_buf<span class="br0">)</span><span class="sy4">;</span>
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wr_stream <span class="sy1"><<</span> <span class="nu0">1234</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</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">read-write stream:
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stringbuf::overflow('4') called
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Before: size of get area: 3
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size of put area: 3
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After : size of get area: 4
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size of put area: 512
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1234
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read-only stream:
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stringbuf::overflow('1') called
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Before: size of get area: 3
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size of put area: 0
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After : size of get area: 3
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size of put area: 0
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write-only stream:
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stringbuf::overflow('4') called
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Before: size of get area: 0
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size of put area: 3
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After : size of get area: 0
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size of put area: 512</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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<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">(<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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<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">(虚受保护成员函数)</span></td>
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</tr>
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<div class="visualClear"></div>
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