163 lines
16 KiB
HTML
163 lines
16 KiB
HTML
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<title>std::ranges::ssize</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_ssize 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::</span>ssize</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></span></div>
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<div>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span> <span class="kw1">namespace</span> <span class="coMULTI">/*unspecified*/</span> <span class="br0">{</span><br /></span>
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<p> <span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> ssize <span class="sy1">=</span> <span class="coMULTI">/*unspecified*/</span><span class="sy4">;</span><br /></p>
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<span class="br0">}</span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(定制点对象)</span></td>
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<div>调用签名</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 /></span>
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<p> requires <span class="coMULTI">/* see below */</span><br /></p>
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<span class="kw4">constexpr</span> <span class="coMULTI">/*signed-integral-like*/</span> ssize<span class="br0">(</span>T<span class="sy3">&&</span> t<span class="br0">)</span><span class="sy4">;</span></div>
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<p>返回转换到有符号类型的范围大小。</p>
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<p>若 <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><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span></span></span> 为良构，令 <code>MadeSigned</code> 代表</p>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span> 的对应有符号版本，若它宽于 <span class="t-lc"><a href="cpp-types-ptrdiff_t.html">std::ptrdiff_t</a></span> ，否则为</li>
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<li><span class="t-lc"><a href="cpp-types-ptrdiff_t.html">std::ptrdiff_t</a></span> ，</li>
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</ul>
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<p>则调用 <code>ranges::ssize</code> 表达式等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span>MadeSigned<span class="sy1">></span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span> 。</p>
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<p>否则，调用 <code>ranges::ssize</code> 为非良构，这能在 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">ssize</span><span class="br0">(</span>t<span class="br0">)</span></span></span> 出现于模板实例化的立即语境时导致<a href="cpp-language-sfinae.html">替换失败</a>。</p>
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<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F.E7.AD.89.E4.BB.B7">表达式等价</span></h3>
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> <i>表达式等价</i>于表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> 拥有相同效果，均为<a href="cpp-language-noexcept_spec.html">潜在抛出</a>或均非潜在抛出（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy1">==</span> <span class="kw1">noexcept</span><span class="br0">(</span>f<span class="br0">)</span></span></span> ），且均为<a href="cpp-language-constant_expression.html">常量子表达式</a>或均非常量子表达式。</p>
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<h3><span class="mw-headline" id=".E5.AE.9A.E5.88.B6.E7.82.B9.E5.AF.B9.E8.B1.A1">定制点对象</span></h3>
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<p>名字 <code>ranges::ssize</code> 代表一个<i>定制点对象</i>，它是<a href="cpp-named_req-LiteralType.html">字面</a> <a href="cpp-concepts-semiregular.html"><code>semiregular</code></a> 类类型的 const <a href="cpp-named_req-FunctionObject.html">函数对象</a>。为说明目的，以 <code><i>__ssize_fn</i></code> 表示其类型的 cv 无限定版本。</p>
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<p><code><i>__ssize_fn</i></code> 的所有实例均相等。在相同参数上调用类型 <code><i>__ssize_fn</i></code> 的不同实例的效果是等价的，无关乎指代该实例的表达式是左值还是右值，以及是否为 const 限定（然而不要求 volatile 限定的实例可调用）。从而能自由地复制 <code>ranges::ssize</code> 并且能彼此替代地使用其副本。</p>
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<p>给定类型集合 <code>Args...</code> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>...</span></span> 满足上面对于 <code>ranges::ssize</code> 的参数要求，则 <code><i>__ssize_fn</i></code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>__ssize_fn, Args...<span class="sy1">></span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span><span class="kw4">const</span> __ssize_fn, Args...<span class="sy1">></span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>__ssize_fn<span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span><span class="kw4">const</span> __ssize_fn<span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 。否则， <code><i>__ssize_fn</i></code> 的函数调用运算符不参与重载决议。</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>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">ssize</span><span class="br0">(</span>e<span class="br0">)</span></span></span> 对于表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 合法，则返回类型为<a href="cpp-iterator-weakly_incrementable.html#.E6.95.B4.E6.95.B0.E5.BC.8F.E7.B1.BB.E5.9E.8B">有符号整数式</a>类型，即 <span class="t-lc"><a href="cpp-types-is_signed.html">std::is_signed_v</a></span> 对其为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 的整数类型，或有符号整数类类型。</p>
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<p>整数式类型的宽度能由 <span class="t-lc"><a href="cpp-types-numeric_limits-digits.html">std::numeric_limits::digits</a></span> 检测。</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 <array></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <ranges></span>
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<span class="co2">#include <type_traits></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-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> arr<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="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> s <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">ssize</span><span class="br0">(</span>arr<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">"ranges::ssize(arr) = "</span> <span class="sy1"><<</span> s <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"ranges::ssize is "</span>
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<span class="sy1"><<</span> <span class="br0">(</span><a href="cpp-types-is_signed.html"><span class="kw517">std::<span class="me2">is_signed_v</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>s<span class="br0">)</span><span class="sy1">></span> <span class="sy4">?</span> <span class="st0">"signed"</span> <span class="sy4">:</span> <span class="st0">"unsigned"</span><span class="br0">)</span>
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<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-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"reversed arr: "</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="sy2">--</span>s<span class="sy4">;</span> s <span class="sy1">>=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="sy2">--</span>s<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> arr<span class="br0">[</span>s<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="st0">"s = "</span> <span class="sy1"><<</span> s <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>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">ranges::ssize(arr) = 5
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ranges::ssize is signed
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reversed arr: 5 4 3 2 1
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s = -1</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://cplusplus.github.io/LWG/issue3403">LWG 3403</a></td>
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<td>C++20</td>
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<td><code>ranges::size</code> 可作用于某些非范围类型但 <code>ranges::ssize</code> 不能</td>
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<td>使之能</td>
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</tr>
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</table>
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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-ranges-size.html"><span class="t-lines"><span>ranges::size</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回等于范围大小的整数<br />
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<span class="t-mark">(定制点对象)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-size.html"><span class="t-lines"><span>size</span><span>ssize</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回容器或数组的大小<br />
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<span class="t-mark">(函数模板)</span></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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