125 lines
9.1 KiB
HTML
125 lines
9.1 KiB
HTML
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<title>std::basic_filebuf<CharT,Traits>::setbuf</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_basic_filebuf_setbuf 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>setbuf</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> <a href="cpp-io-basic_streambuf.html"><span class="kw1666">std::<span class="me2">basic_streambuf</span></span></a><span class="sy1"><</span>CharT, Traits<span class="sy1">></span><span class="sy2">*</span> setbuf<span class="br0">(</span> char_type<span class="sy2">*</span> s, <a href="cpp-io-streamsize.html"><span class="kw1804">std::<span class="me2">streamsize</span></span></a> n <span class="br0">)</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<td></td>
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<td></td>
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<p>若 <code>s</code> 为空指针且 <code>n</code> 为零，则 filebuf 变为对输出<i>无缓冲</i>，这表示 <code>pbase()</code> 和 <code>pptr()</code> 为空，而任何输出都被立即发送到文件。</p>
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<p>否则，如同调用 <code>setbuf()</code> ，以用户提供的首元素为<code>s</code> 所指向的字符数组替换内部缓冲区（受控制字符序列），并允许此 <span class="t-lc"><a href="cpp-io-basic_filebuf.html">std::basic_filebuf</a></span> 将该数组中的至多 <code>n</code> 个字节用于缓冲。</p>
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<p>此函数为受保护虚，它仅可通过 <code>pubsetbuf()</code> 或从导出自 <code>std::basic_filebuf</code> 的用户定义类调用。</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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<tr class="t-par">
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<td>s</td>
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<td>-</td>
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<td>指向用户提供缓冲区的首个 CharT 的指针或空指针</td>
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</tr>
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<tr class="t-par">
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<td>n</td>
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<td>-</td>
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<td>用户提供缓冲区中的 CharT 元素数或零</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><span class="t-c"><span class="mw-geshi cpp source-cpp">this</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>可以使用此函数时的条件，和使用提供的缓冲区的方式是实现定义的。</p>
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<ul>
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<li>GCC 4.6 libstdc++</li>
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</ul>
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<dl>
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<dd><code>setbuf()</code> 仅可在 <span class="t-lc"><a href="cpp-io-basic_filebuf.html">std::basic_filebuf</a></span> 未与文件关联时调用（否则无效果）。拥有用户提供的缓冲区时，每次从文件读取 <code>n-1</code> 字节。</dd>
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</dl>
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<ul>
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<li>Clang++3.0 libc++</li>
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</ul>
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<dl>
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<dd><code>setbuf()</code> 可在打开文件后，但要在任何 I/O 前调用（否则可能崩溃）。拥有用户提供缓冲区时，从文件读取适合缓冲区的 4096 最大倍数字节。</dd>
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</dl>
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<ul>
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<li>Visual Studio 2010</li>
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</ul>
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<dl>
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<dd><code>setbuf()</code> 可在任何时候调用，即使在某个 I/O 发生后。缓冲区的当前内容若存在则丢失。</dd>
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</dl>
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<p>标准不定义此函数的任何行为，除了要求在任何 I/O 发生前调用的 <code>setbuf(0, 0)</code> 设置输出为无缓冲。</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>为读取提供 10k 缓冲区。 Linux 上，可用 strace 工具观察实际读取的字节数</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 <fstream></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <string></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">int</span> cnt <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<a href="cpp-io-basic_ifstream.html"><span class="kw1732">std::<span class="me2">ifstream</span></span></a> file<span class="sy4">;</span>
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<span class="kw4">char</span> buf<span class="br0">[</span><span class="nu0">10241</span><span class="br0">]</span><span class="sy4">;</span>
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file.<span class="me1">rdbuf</span><span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>pubsetbuf<span class="br0">(</span>buf, sizeof buf<span class="br0">)</span><span class="sy4">;</span>
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file.<span class="me1">open</span><span class="br0">(</span><span class="st0">"/usr/share/dict/words"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> line<span class="sy4">;</span> getline<span class="br0">(</span>file, line<span class="br0">)</span><span class="sy4">;</span> <span class="br0">)</span>
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<span class="sy2">++</span>cnt<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> cnt <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></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-pubsetbuf.html"><span class="t-lines"><span>pubsetbuf</span></span></a></div>
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</div>
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</td>
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<td>调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">setbuf<span class="br0">(</span><span class="br0">)</span></span></span><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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<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-c-setvbuf.html"><span class="t-lines"><span>setvbuf</span></span></a></div>
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</div>
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</td>
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<td>为文件流设置缓冲区与其大小<br />
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<span class="t-mark">(函数)</span></td>
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</div>
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<div class="visualClear"></div>
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</div>
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