117 lines
12 KiB
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117 lines
12 KiB
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<title>std::begin(std::valarray)</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>begin<small>(std::valarray)</small></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-valarray.html">valarray</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="coMULTI">/*unspecified1*/</span> begin<span class="br0">(</span> valarray<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> v <span class="br0">)</span><span class="sy4">;</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">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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<span class="coMULTI">/*unspecified2*/</span> begin<span class="br0">(</span> <span class="kw4">const</span> valarray<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> v <span class="br0">)</span><span class="sy4">;</span></span></div>
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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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<p><span class="t-lc"><a href="cpp-iterator-begin.html">std::begin</a></span> 对 <code>valarray</code> 的重载返回指代数值数组中首元素的未指定类型迭代器。</p>
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<div class="t-li1"><span class="t-li">1)</span> 返回类型满足可变<a href="cpp-named_req-RandomAccessIterator.html"><span style="font-style:italic">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</span></a> 的要求。</div>
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<div class="t-li1"><span class="t-li">2)</span> 返回类型满足常<a href="cpp-named_req-RandomAccessIterator.html"><span style="font-style:italic">老式随机访问迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyRandomAccessIterator)</span></a> 的要求。</div>
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<p>在数组 <code>v</code> 上调用成员函数 <span class="t-lc">resize()</span> ，或在 <code>v</code> 的生存期结束，两者之一到来时，从此函数获得的迭代器被非法化。</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>v</td>
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<td>-</td>
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<td>数值数组</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>指向数值数组中首个值的迭代器。</p>
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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>可能抛出实现定义的异常。</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>不同于其他接收 <code>std::valarray</code> 参数的函数， <code>begin()</code> 不能接受可从涉及 valarray 的表达式返回的替换类型（例如表达式模板所产生的类型）： <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-begin.html"><span class="kw676">std::<span class="me2">begin</span></span></a><span class="br0">(</span>v1 <span class="sy2">+</span> v2<span class="br0">)</span></span></span> 不可移植，必须用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-iterator-begin.html"><span class="kw676">std::<span class="me2">begin</span></span></a><span class="br0">(</span><a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>v1 <span class="sy2">+</span> v2<span class="br0">)</span><span class="br0">)</span></span></span> 代替。</p>
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<p>此函数的意图是允许<a href="cpp-language-range-for.html">范围 for 循环</a>能作用于 valarray ，而非提供容器语义。</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 class="t-example-live-link"></div>
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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 <valarray></span>
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<span class="co2">#include <algorithm></span>
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<span class="kw4">auto</span> show <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> <span class="kw4">const</span><span class="sy3">&</span> v<span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-algorithm-for_each.html"><span class="kw1568">std::<span class="me2">for_each</span></span></a><span class="br0">(</span><a href="cpp-iterator-begin.html"><span class="kw676">std::<span class="me2">begin</span></span></a><span class="br0">(</span>v<span class="br0">)</span>, <a href="cpp-iterator-end.html"><span class="kw680">std::<span class="me2">end</span></span></a><span class="br0">(</span>v<span class="br0">)</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> c<span class="br0">)</span> <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> c <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<span class="br0">}</span><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">'<span class="es1">\n</span>'</span><span class="sy4">;</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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<span class="kw4">const</span> <a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> x <span class="br0">{</span> <span class="nu0">47</span>, <span class="nu0">70</span>, <span class="nu0">37</span>, <span class="nu0">52</span>, <span class="nu0">90</span>, <span class="nu0">23</span>, <span class="nu0">17</span>, <span class="nu0">33</span>, <span class="nu0">22</span>, <span class="nu0">16</span>, <span class="nu0">21</span>, <span class="nu0">4</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">const</span> <a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> y <span class="br0">{</span> <span class="nu0">25</span>, <span class="nu0">31</span>, <span class="nu0">71</span>, <span class="nu0">56</span>, <span class="nu0">21</span>, <span class="nu0">21</span>, <span class="nu0">15</span>, <span class="nu0">34</span>, <span class="nu0">21</span>, <span class="nu0">27</span>, <span class="nu0">12</span>, <span class="nu0">6</span> <span class="br0">}</span><span class="sy4">;</span>
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show<span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span>
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show<span class="br0">(</span>y<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">const</span> <a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> z <span class="br0">{</span> x <span class="sy2">+</span> y <span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-algorithm-for_each.html"><span class="kw1568">std::<span class="me2">for_each</span></span></a><span class="br0">(</span><a href="cpp-iterator-begin.html"><span class="kw676">std::<span class="me2">begin</span></span></a><span class="br0">(</span>z<span class="br0">)</span>, <a href="cpp-iterator-end.html"><span class="kw680">std::<span class="me2">end</span></span></a><span class="br0">(</span>z<span class="br0">)</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">char</span> c<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> c<span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</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>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">47 70 37 52 90 23 17 33 22 16 21 4
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25 31 71 56 21 21 15 34 21 27 12 6
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Hello, C++!</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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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-valarray-end2.html"><span class="t-lines"><span>std::end<span class="t-dsc-small">(std::valarray)</span></span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>特化的 <span class="t-lc"><a href="cpp-iterator-end.html">std::end</a></span><br />
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<span class="t-mark">(函数模板)</span></td>
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