138 lines
13 KiB
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138 lines
13 KiB
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<title>std::ranges::take_view<V>::take_view</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_take_view_take_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::take_view<V>::</span>take_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-take_view.html">take view</a></span></div>
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<div><span class="mw-geshi cpp source-cpp">take_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="sy1">=</span> <span class="kw1">default</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-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> take_view<span class="br0">(</span> V base, ranges<span class="sy4">::</span><span class="me2">range_difference_t</span><span class="sy1"><</span>V<span class="sy1">></span> count <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-cxx20">(C++20 起)</span></td>
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<td></td>
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<td></td>
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<td></td>
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<p>构造 <code>take_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>底层视图并初始化计数为 0 。构造后 <a href="cpp-ranges-take_view-base.html"><code>base()</code></a> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">V<span class="br0">(</span><span class="br0">)</span></span></span> 的副本而 <a href="cpp-ranges-take_view-size.html"><code>size()</code></a> 返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">​<span class="nu0">0</span>​</span></span> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 以 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>base<span class="br0">)</span></span></span> 初始化底层视图并以 <code>count</code> 初始化计数。构造后 <a href="cpp-ranges-take_view-base.html"><code>base()</code></a> 返回 <code>base</code> 的副本而 <a href="cpp-ranges-take_view-size.html"><code>size()</code></a> 返回 <code>count</code> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span>base<span class="br0">)</span></span></span> 的较小者。</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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<td>base</td>
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<td>-</td>
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<td>底层视图</td>
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</tr>
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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>要收取的元素数</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>打印首 <code>n</code> 个质数，用<a href="https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes" class="extiw" title="enwiki:Sieve of Eratosthenes">埃拉托色尼筛</a>法生成起始。</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 <bit></span>
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<span class="co2">#include <bitset></span>
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<span class="co2">#include <iomanip></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <limits></span>
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<span class="co2">#include <ranges></span>
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<span class="kw4">constexpr</span> <span class="kw4">unsigned</span> clog2<span class="br0">(</span><span class="kw4">auto</span> x<span class="br0">)</span> <span class="co1">// ~ ⌈ log₂(x) ⌉</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>x<span class="br0">)</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">digits</span> <span class="sy2">-</span> std<span class="sy4">::</span><span class="me2">countl_zero</span><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw4">unsigned</span> Primes<span class="sy1">></span> <span class="kw1">struct</span> Sieve
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<span class="br0">{</span>
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<span class="kw4">bool</span> operator<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>
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<span class="kw1">if</span> <span class="br0">(</span>n <span class="sy1"><</span> <span class="nu0">2</span><span class="br0">)</span> <span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>n <span class="sy1">==</span> <span class="nu0">2</span><span class="br0">)</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>n <span class="sy2">%</span> <span class="nu0">2</span> <span class="sy1">==</span> <span class="nu0">0</span> <span class="sy3">||</span> bits.<span class="me1">test</span><span class="br0">(</span>n <span class="sy2">/</span> <span class="nu0">2</span><span class="br0">)</span><span class="br0">)</span>
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<span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i<span class="br0">{</span>n<span class="br0">}</span>, j<span class="br0">{</span><span class="nu0">3</span><span class="br0">}</span>, k<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">(</span>k <span class="sy1">=</span> i <span class="sy2">*</span> j<span class="br0">)</span> <span class="sy1"><</span> size<span class="sy4">;</span> j <span class="sy2">+</span><span class="sy1">=</span> <span class="nu0">2</span><span class="br0">)</span>
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bits.<span class="me1">set</span><span class="br0">(</span>k <span class="sy2">/</span> <span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">static</span> <span class="kw4">constexpr</span> <span class="kw4">int</span> prime_numbers <span class="sy1">=</span> Primes<span class="sy4">;</span>
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<span class="co1">// a(n) ~= n*(log(n) + log(log(n)));</span>
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<span class="kw4">static</span> <span class="kw4">constexpr</span> <span class="kw4">int</span> size <span class="sy1">=</span> Primes<span class="sy2">*</span><span class="br0">(</span>clog2<span class="br0">(</span>Primes<span class="br0">)</span> <span class="sy2">+</span> clog2<span class="br0">(</span>clog2<span class="br0">(</span>Primes<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 仅保留奇数； 0 意味着它是质数</span>
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<span class="kw1">private</span><span class="sy4">:</span> <a href="cpp-utility-bitset.html"><span class="kw1027">std::<span class="me2">bitset</span></span></a><span class="sy1"><</span>size <span class="sy2">/</span> <span class="nu0">2</span> <span class="sy2">+</span> <span class="nu0">1</span><span class="sy1">></span> bits<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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Sieve<span class="sy1"><</span><span class="nu0">100</span><span class="sy1">></span> sieve<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> primes_gen <span class="sy1">=</span> std<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">1</span><span class="br0">)</span>
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<span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">filter</span><span class="br0">(</span>sieve<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> primes <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">take_view</span><span class="br0">{</span> primes_gen, sieve.<span class="me1">prime_numbers</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">int</span> n<span class="br0">{</span><span class="nu0">1</span><span class="br0">}</span><span class="sy4">;</span> <span class="kw4">auto</span> prime <span class="sy4">:</span> primes<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> prime <span class="sy1"><<</span> <span class="br0">(</span>n<span class="sy2">++</span> <span class="sy2">%</span> <span class="nu0">10</span> <span class="sy4">?</span> <span class="st0">' '</span> <span class="sy4">:</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="br0">)</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>
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<span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span> <span class="st0">"sieve.prime_numbers: "</span> <span class="sy1"><<</span> sieve.<span class="me1">prime_numbers</span>
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<span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span> <span class="st0">"sieve.size: "</span> <span class="sy1"><<</span> sieve.<span class="me1">size</span>
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<span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span> <span class="st0">"sizeof(sieve): "</span> <span class="sy1"><<</span> sizeof sieve <span class="sy1"><<</span> <span class="st0">" bytes<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>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">2 3 5 7 11 13 17 19 23 29
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31 37 41 43 47 53 59 61 67 71
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73 79 83 89 97 101 103 107 109 113
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127 131 137 139 149 151 157 163 167 173
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179 181 191 193 197 199 211 223 227 229
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233 239 241 251 257 263 269 271 277 281
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283 293 307 311 313 317 331 337 347 349
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353 359 367 373 379 383 389 397 401 409
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419 421 431 433 439 443 449 457 461 463
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467 479 487 491 499 503 509 521 523 541
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sieve.prime_numbers: 100
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sieve.size: 1000
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sizeof(sieve): 64 bytes</pre></div>
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