123 lines
9.6 KiB
HTML
123 lines
9.6 KiB
HTML
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<title>std::ranges::transform_view<V,F>::transform_view</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_transform_view_transform_view 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::ranges::transform_view<V,F>::</span>transform_view</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-ranges.html">ranges</a>‎ | <a href="cpp-ranges-transform_view.html">transform view</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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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp">transform_view<span class="br0">(</span><span class="br0">)</span> requires std<span class="sy4">::</span><span class="me2">default_initializable</span><span class="sy1"><</span>V<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">default_initializable</span><span class="sy1"><</span>F<span class="sy1">></span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl t-since-cxx20">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> transform_view<span class="br0">(</span> V base, F fun <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-cxx20">(C++20 起)</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tr>
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</tbody>
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</table>
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<p>构造 <code>transform_view</code> 。</p>
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<div class="t-li1"><span class="t-li">1)</span> 默认构造函数。<a href="cpp-language-value_initialization.html">值初始化</a>底层视图与变换函数。</div>
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<div class="t-li1"><span class="t-li">2)</span> 从 <code>base</code> 移动构造视图并从 <code>fun</code> 移动构造变换函数。</div>
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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>base</td>
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<td>-</td>
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<td>视图</td>
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</tr>
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<tr class="t-par">
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<td>fun</td>
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<td>-</td>
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<td>变换函数</td>
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</tr>
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</table>
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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>演示使用 1 的反正切的级数展开的 π 近似：<br />
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<code>atan(1) = π/4 ~ 1 - 1/3 + 1/5 - 1/7 + 1/9 - 1/11 + ...</code></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 <iomanip></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <numbers></span>
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<span class="co2">#include <ranges></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-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-setprecision.html"><span class="kw1796">std::<span class="me2">setprecision</span></span></a><span class="br0">(</span><span class="nu0">15</span><span class="br0">)</span> <span class="sy1"><<</span> <a href="cpp-io-manip-fixed.html"><span class="kw1784">std::<span class="me2">fixed</span></span></a><span class="sy4">;</span>
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<span class="kw4">auto</span> atan1term <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">transform</span><span class="br0">(</span>
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<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="br0">(</span><span class="br0">(</span>n <span class="sy2">%</span> <span class="nu0">2</span><span class="br0">)</span> <span class="sy4">?</span> <span class="sy2">-</span><span class="nu0">1</span> <span class="sy4">:</span> <span class="nu0">1</span><span class="br0">)</span> <span class="sy2">*</span> <span class="nu16">1.0</span> <span class="sy2">/</span> <span class="br0">(</span><span class="nu0">2</span> <span class="sy2">*</span> n <span class="sy2">+</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span> iterations <span class="sy4">:</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">10</span>, <span class="nu0">100</span>, <span class="nu0">1000</span>, <span class="nu0">1</span><span class="st0">'000'</span><span class="nu8">000</span><span class="br0">}</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">double</span> accum<span class="br0">{</span><span class="nu16">0.0</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">double</span> term <span class="sy4">:</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">iota</span><span class="br0">(</span><span class="nu0">0</span>, iterations<span class="br0">)</span> <span class="sy3">|</span> atan1term<span class="br0">)</span> <span class="br0">{</span>
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accum <span class="sy2">+</span><span class="sy1">=</span> term<span class="sy4">;</span>
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<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> <span class="st0">"π ~ "</span> <span class="sy1"><<</span> <span class="nu0">4</span> <span class="sy2">*</span> accum <span class="sy1"><<</span> <span class="st0">" (iterations: "</span> <span class="sy1"><<</span> iterations <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>
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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">"π ~ "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">numbers</span><span class="sy4">::</span><span class="me2">pi</span> <span class="sy1"><<</span> <span class="st0">" (std::numbers::pi)<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">π ~ 4.000000000000000 (iterations: 1)
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π ~ 2.666666666666667 (iterations: 2)
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π ~ 3.466666666666667 (iterations: 3)
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π ~ 2.895238095238096 (iterations: 4)
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π ~ 3.339682539682540 (iterations: 5)
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π ~ 3.041839618929403 (iterations: 10)
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π ~ 3.131592903558554 (iterations: 100)
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π ~ 3.140592653839794 (iterations: 1000)
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π ~ 3.141591653589774 (iterations: 1000000)
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π ~ 3.141592653589793 (std::numbers::pi)</pre></div>
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</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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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P2325R3">P2325R3</a></td>
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<td>C++20</td>
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<td>若 <code>F</code> 非 <a href="cpp-concepts-default_initializable.html"><code>default_initializable</code></a> ，则默认构造函数构造不含 <code>F</code> 的 <code>transform_view</code></td>
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<td><code>transform_view</code> 亦非<br />
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<a href="cpp-concepts-default_initializable.html"><code>default_initializable</code></a></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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</body>
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