176 lines
11 KiB
HTML
176 lines
11 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::freopen</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_c_freopen skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>freopen</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-c.html">c</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-cstdio.html"><cstdio></a></code></div>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-io-c.html"><span class="kw1876">std::<span class="me2">FILE</span></span></a><span class="sy2">*</span> freopen<span class="br0">(</span> <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> filename, <span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> mode, <a href="cpp-io-c.html"><span class="kw1876">std::<span class="me2">FILE</span></span></a><span class="sy2">*</span> stream <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<p>首先，试图关闭与 <code>stream</code> 关联的文件，忽略任何错误。然后，若 <code>filename</code> 非空，则试图用 <code>mode</code> 打开 <code>filename</code> 所指定的文件，如同用 <span class="t-lc">fopen</span> ，然后将该文件与 <code>stream</code> 所指向的文件流关联。若 <code>filename</code> 为空指针，则函数试图重打开已与 <code>stream</code> 关联的文件（此情况下是否允许模式改变是实现定义的）。</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>filename</td>
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<td>-</td>
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<td>要关联文件流到的文件</td>
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</tr>
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<tr class="t-par">
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<td>mode</td>
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<td>-</td>
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<td>确定新文件打开模式的空终止字符串
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<table class="wikitable">
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<tr>
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<th>文件访问模式字符串</th>
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<th>含义</th>
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<th>解释</th>
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<th>若文件已存在的行动</th>
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<th>若文件不存在的行动</th>
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<td><code>"r"</code></td>
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<td>读</td>
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<td>为读取打开文件</td>
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<td>从起始读取</td>
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<td>打开失败</td>
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</tr>
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<tr>
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<td><code>"w"</code></td>
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<td>写</td>
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<td>为写入创建文件</td>
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<td>销毁内容</td>
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<td>创建新文件</td>
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</tr>
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<td><code>"a"</code></td>
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<td>追加</td>
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<td>追加到文件</td>
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<td>写入到末尾</td>
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<td>创建新文件</td>
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</tr>
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<td><code>"r+"</code></td>
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<td>扩展读</td>
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<td>为读取/写入打开文件</td>
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<td>从起始读取</td>
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<td>错误</td>
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</tr>
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<tr>
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<td><code>"w+"</code></td>
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<td>扩展写</td>
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<td>为读取/写入创建文件</td>
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<td>销毁内容</td>
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<td>创建新文件</td>
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</tr>
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<tr>
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<td><code>"a+"</code></td>
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<td>扩展追加</td>
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<td>为读取/写入打开文件</td>
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<td>写入到末尾</td>
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<td>创建新文件</td>
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<td>文件访问模式标志 <code>"b"</code> 能可选地指定，以<a href="cpp-io-c-FILE.html#.E4.BA.8C.E8.BF.9B.E5.88.B6.E4.B8.8E.E6.96.87.E6.9C.AC.E6.A8.A1.E5.BC.8F">用二进制模式</a>打开文件。此标志在 POSIX 系统上无效果，但例如在 Windows 上，它禁用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">'<span class="es1">\n</span>'</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">'<span class="es2">\x1A</span>'</span></span></span> 的特殊处理。<br />
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在追加文件访问模式上，写入数据到文件尾，忽略文件位置指示器的当前位置。</td>
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<td>文件访问模式标志 <code>"x"</code> 能可选地追加到 "w" 或 "w+" 指定符。此标志强制函数在文件存在的情况下失败，而非重写文件。<span class="t-mark-rev t-since-cxx17">(C++17)</span></td>
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<td>若模式不是以上字符串之一，则行为未定义。一些实现定义额外的支持模式（例如 <a rel="nofollow" class="external text" href="https://docs.microsoft.com/zh-cn/cpp/c-runtime-library/reference/fopen-wfopen">MSVC</a> ）。</td>
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</table>
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</td>
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<tr class="t-par">
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<td>stream</td>
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<td>-</td>
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<td>要修改的文件流</td>
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</tr>
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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 <code>stream</code> ，失败时为空指针。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p><code>freopen</code> 是一旦由 I/O 操作或 <span class="t-lc">std::fwide</span> 建立面向后，改变流窄/宽面向的唯一方式。</p>
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<p><code>freopen</code> 的 Microsoft CRT 版本在 <code>filename</code> 为空指针时不支持任何模式更改并将它当作错误（见<a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/freopen-wfreopen">文档</a>）。可行的替代方案是非标准函数 <a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/setmode"><code>_setmode()</code></a> 。</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>下列代码重定向 <code>stdout</code> 到文件</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 <cstdio></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-c-fprintf.html"><span class="kw1844">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"stdout is printed to console<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">freopen</span><span class="br0">(</span><span class="st0">"redir.txt"</span>, <span class="st0">"w"</span>, <span class="kw2">stdout</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-io-c-fprintf.html"><span class="kw1844">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"stdout is redirected to a file<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 这被写入 redir.txt</span>
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<a href="cpp-io-c-fclose.html"><span class="kw1815">std::<span class="me2">fclose</span></span></a><span class="br0">(</span><span class="kw2">stdout</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</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">stdout is printed to console</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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<table class="t-dsc-begin">
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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-fopen.html"><span class="t-lines"><span>fopen</span></span></a></div>
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</div>
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</td>
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<td>打开文件<br />
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<span class="t-mark">(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-c-fclose.html"><span class="t-lines"><span>fclose</span></span></a></div>
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</div>
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</td>
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<td>关闭文件<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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