110 lines
11 KiB
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110 lines
11 KiB
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<title>std::random_device::random_device</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::random_device::</span>random_device</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-numeric.html">numeric</a>‎ | <a href="cpp-numeric-random.html">random</a>‎ | <a href="cpp-numeric-random-random_device.html">random device</a></span></div>
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<table class="t-dcl-begin">
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<div><span class="mw-geshi cpp source-cpp">random_device<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> random_device<span class="br0">(</span><span class="coMULTI">/*implementation-defined*/</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span> random_device<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> token<span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp">random_device<span class="br0">(</span><span class="kw4">const</span> random_device<span class="sy3">&</span> <span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<td></td>
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<div class="t-li1"><span class="t-li">1)</span> 以实现定义的 <code>token</code> 默认构造新的 <span class="t-lc"><a href="cpp-numeric-random-random_device.html">std::random_device</a></span> 对象。</div>
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<div class="t-li1"><span class="t-li">2)</span> 构造新的 <span class="t-lc"><a href="cpp-numeric-random-random_device.html">std::random_device</a></span> 对象，若提供则以实现定义行为利用参数 <code>token</code> 。</div>
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<div class="t-li1"><span class="t-li">3)</span> 复制构造函数被删除： <code>std::random_device</code> 不可复制。</div>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>失败时抛出从 <span class="t-lc"><a href="cpp-error-exception.html">std::exception</a></span> 派生的实现定义异常。</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><a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B11/random.cc">libstdc++</a> 中的实现期待 <code>token</code> 指名随机字节的源。可能的记号值包括 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"default"</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"rand_s"</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"rdseed"</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"rdrand"</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"rdrnd"</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"/dev/urandom"</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"/dev/random"</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"mt19937"</span></span></span> 和指定 mt19937 引擎的整数字符串。（除了 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"default"</span></span></span> 的记号仅在某些目标上可用。）</p>
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<p><a rel="nofollow" class="external text" href="http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/random.cpp?view=markup">libc++</a> 中的实现在配置以字符设备为源时，期待 <code>token</code> 为在从它读取时产生随机数的字符设备的名字；否则它期待 <code>token</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"/dev/urandom"</span></span></span> 。</p>
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<p>若提供不支持的记号则 libstdc++ 与 libc++ 均抛出异常。 <a rel="nofollow" class="external text" href="https://github.com/microsoft/STL/blob/c10ae01b4d9508eed9d5f059a120ee7223b6ac12/stl/inc/random#L5026">Microsoft STL</a> 完全忽略记号。</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>演示 Linux 上 <code>std::random_device</code> 二种常见的可用类型</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 <iostream></span>
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<span class="co2">#include <random></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-numeric-random-uniform_int_distribution.html"><span class="kw1502">std::<span class="me2">uniform_int_distribution</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> d<span class="br0">(</span><span class="nu0">0</span>, <span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-numeric-random-random_device.html"><span class="kw1486">std::<span class="me2">random_device</span></span></a> rd1<span class="sy4">;</span> <span class="co1">// 使用 RDRND 或 /dev/urandom</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n<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> d<span class="br0">(</span>rd1<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</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>'</span><span class="sy4">;</span>
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<a href="cpp-numeric-random-random_device.html"><span class="kw1486">std::<span class="me2">random_device</span></span></a> rd2<span class="br0">(</span><span class="st0">"/dev/random"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Linux 上更慢</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> <span class="nu0">10</span><span class="sy4">;</span> <span class="sy2">++</span>n<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> d<span class="br0">(</span>rd2<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</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>'</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">7 10 7 0 4 4 6 9 4 7
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2 4 10 6 3 2 0 6 3 7</pre></div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P0935R0">P0935R0</a></td>
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<td>C++11</td>
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<td>默认构造函数为 explicit</td>
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<td>使之为隐式</td>
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</div>
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<div class="visualClear"></div>
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