137 lines
9.3 KiB
HTML
137 lines
9.3 KiB
HTML
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<title>std::fread</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_io_c_fread 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::</span>fread</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>定义于头文件 <code><a href="cpp-header-cstdio.html"><cstdio></a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> fread<span class="br0">(</span> <span class="kw4">void</span><span class="sy2">*</span> buffer, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> size, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> count, <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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<p>从给定输入流 <code>stream</code> 读取至多 <code>count</code> 个对象到数组 <code>buffer</code> 中，如同以对每个对象调用 <code>size</code> 次 <span class="t-lc"><a href="cpp-io-c-fgetc.html">std::fgetc</a></span> ，并按顺序存储结果到转译为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> 数组的 <code>buffer</code> 中的相继位置。流的文件位置指示器前进读取的字符数。</p>
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<p>若对象不<a href="cpp-named_req-TriviallyCopyable.html"><span style="font-style:italic">可平凡复制</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(TriviallyCopyable)</span></a> ，则行为未定义。</p>
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<p>若出现错误，则流的文件位置指示器的结果值不确定。若读入部分的元素，则元素值不确定。</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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<td>buffer</td>
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<td>-</td>
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<td>指向要读取的数组中首个对象的指针</td>
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<td>size</td>
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<td>-</td>
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<td>每个对象的字节大小</td>
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<tr class="t-par">
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<td>count</td>
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<td>-</td>
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<td>要读取的对象数</td>
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<td>stream</td>
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<td>-</td>
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<td>读取来源的输入文件流</td>
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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>成功读取的对象数，若出现错误或文件尾条件，则可能小于 <code>count</code> 。</p>
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<p>若 <code>size</code> 或 <code>count</code> 为零，则 <code>fread</code> 返回零且不进行其他动作。</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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<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 <iostream></span>
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<span class="co2">#include <cstdio></span>
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<span class="co2">#include <fstream></span>
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<span class="co2">#include <vector></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="co1">// 准备文件</span>
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<a href="cpp-io-basic_ofstream.html"><span class="kw1733">std::<span class="me2">ofstream</span></span></a><span class="br0">(</span><span class="st0">"test.txt"</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="nu0">1</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <span class="nu0">2</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-c.html"><span class="kw1876">std::<span class="me2">FILE</span></span></a><span class="sy2">*</span> f <span class="sy1">=</span> <a href="cpp-io-c-fopen.html"><span class="kw1813">std::<span class="me2">fopen</span></span></a><span class="br0">(</span><span class="st0">"test.txt"</span>, <span class="st0">"r"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span> buf<span class="br0">(</span><span class="nu0">4</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// char 可平凡复制</span>
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std<span class="sy4">::</span><span class="me2">fread</span><span class="br0">(</span><span class="sy3">&</span>buf<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>, sizeof buf<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span>, buf.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span>, f<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> n <span class="sy4">:</span> buf<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> n<span class="sy4">;</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> buf2<span class="sy4">;</span> <span class="co1">// string 不可平凡复制</span>
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<span class="co1">// 这会导致未定义行为</span>
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<span class="co1">// std::fread(&buf2[0], sizeof buf2[0], buf2.size(), f);</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">1 2</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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<div class="t-dsc-member-div">
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<div><a href="cpp-io-c-fscanf.html"><span class="t-lines"><span>scanf</span><span>fscanf</span><span>sscanf</span></span></a></div>
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<td>从 <span class="t-lc"><a href="cpp-io-c.html">stdin</a></span>、文件流或缓冲区读取有格式输入<br />
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<span class="t-mark">(函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-c-fgets.html"><span class="t-lines"><span>fgets</span></span></a></div>
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</td>
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<td>从文件流获取字符串<br />
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<span class="t-mark">(函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-io-c-fwrite.html"><span class="t-lines"><span>fwrite</span></span></a></div>
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<td>写入文件<br />
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<span class="t-mark">(函数)</span></td>
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<div class="visualClear"></div>
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