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<div class="identifier-type">function</div><div class="header"><span></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="class" title="class span">span::</span><span class="token">subspan</span><span class="cpp cpp20" title="C++20で追加">(C++20)</span></h1>
<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="../../cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">Offset</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../cstddef/size_t.html">std::size_t</a></span><span class="w"> </span><span class="n">Count</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../dynamic_extent.html">dynamic_extent</a></span><span class="o">></span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">span</span><span class="o"><</span><span class="n">element_type</span><span class="p">,</span><span class="w"> </span><span class="n"><i>see below</i></span><span class="o">></span>
<span class="w"> </span><span class="n">subspan</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="p">;</span><span class="w"> </span><span class="c1">// (1)</span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">span</span><span class="o"><</span><span class="n">element_type</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../dynamic_extent.html">dynamic_extent</a></span><span class="o">></span>
<span class="w"> </span><span class="n">subspan</span><span class="p">(</span><span class="n">size_type</span><span class="w"> </span><span class="n">offset</span><span class="p">,</span><span class="w"> </span><span class="n">size_type</span><span class="w"> </span><span class="n">count</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../dynamic_extent.html">dynamic_extent</a></span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="p">;</span><span class="w"> </span><span class="c1">// (2)</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>任意の位置からN個の要素を参照する<code>span</code>オブジェクトを取得する。</p>
<ul>
<li>(1) : テンプレートパラメータで指定された任意の開始位置<code>Offset</code>と要素数<code>Count</code>にしたがって要素を取り出す</li>
<li>(2) : パラメータで指定された任意の開始位置<code>offset</code>と要素数<code>count</code>にしたがって要素を取り出す</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Mandates。満たさなければプログラムが不適格となる要件。この要件は、オーバーロード解決によって選択された関数内での`static_assert`の定数条件式や、関数に対するdelete宣言などで表現できる">適格要件</a></h2>
<ul>
<li>(1) : <code>Offset <= Extent && (Count == <a href="../dynamic_extent.html">dynamic_extent</a> || Count <= Extent - Offset)</code>が<code>true</code>であること</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Hardened preconditions。関数呼び出し時に満たされていると関数が想定する条件。満たさない場合は契約違反となる動作。セキュリティ上の脆弱性につながる操作に対して設定される事前条件">堅牢化された事前条件</a></h2>
<p>(1)であれば<code>Offset</code>を<code>i</code>、<code>Count</code>を<code>N</code>とし、(2)であれば<code>offset</code>を<code>i</code>、<code>count</code>を<code>N</code>として、</p>
<ul>
<li><code>i <= <a href="size.html">size()</a> && (N == <a href="../dynamic_extent.html">dynamic_extent</a> || N <= <a href="size.html">size()</a> - i)</code>が<code>true</code>であること</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a></h2>
<ul>
<li>
<p>(1) : 以下と等価：
<br />
<div class="codehilite"><pre><span></span><code><span class="k">return</span><span class="w"> </span><span class="n">span</span><span class="o"><</span><span class="n">ElementType</span><span class="p">,</span><span class="w"> </span><span class="n"><i>see below</i></span><span class="o">></span><span class="p">(</span>
<span class="w"> </span><span class="n"><a href="data.html">data()</a></span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">Offset</span><span class="p">,</span><span class="w"> </span><span class="n">Count</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n"><a href="../dynamic_extent.html">dynamic_extent</a></span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">Count</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n"><a href="size.html">size()</a></span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">Offset</span><span class="p">);</span>
</code></pre></div>
</p>
<ul>
<li>ここで<a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a>の型の第2テンプレート引数は以下になる：</li>
</ul>
<p><div class="codehilite"><pre><span></span><code><span class="n">Count</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">dynamic_extent</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">Count</span><span class="w"> </span><span class="o">:</span>
<span class="p">(</span><span class="n">Extent</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">dynamic_extent</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">Extent</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">Offset</span><span class="w"> </span><span class="o">:</span>
<span class="n">dynamic_extent</span><span class="p">)</span>
</code></pre></div>
</p>
</li>
<li>
<p>(2) : 以下と等価：
<br />
<div class="codehilite"><pre><span></span><code><span class="k">return</span><span class="w"> </span><span class="n">span</span><span class="o"><</span><span class="n">element_type</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../dynamic_extent.html">dynamic_extent</a></span><span class="o">></span><span class="p">(</span>
<span class="w"> </span><span class="n"><a href="data.html">data()</a></span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">offset</span><span class="p">,</span><span class="w"> </span><span class="n">count</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n"><a href="../dynamic_extent.html">dynamic_extent</a></span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n"><a href="size.html">size()</a></span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">offset</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">count</span><span class="p">);</span>
</code></pre></div>
</p>
</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Complexity。関数が使うアルゴリズムの空間計算量・時間計算量">計算量</a></h2>
<p>定数時間</p>
<h2>例</h2>
<p><div class="yata" id="0cd23f86d7ffa5a2e4f4859b97af5608dc9ebe8c"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../span.html"><span></a></span>
<span class="cp">#include <a href="../../vector.html"><vector></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">};</span>
<span class="w"> </span><span class="n"><a href="../span.html">std::span</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../span.html">std::span</a></span><span class="p">{</span><span class="n">v</span><span class="p">};</span>
<span class="w"> </span><span class="c1">// (1) : テンプレート引数として開始位置と要素数を指定して、要素を取得する。</span>
<span class="w"> </span><span class="c1">// テンプレート内でこのオーバーロードを使用する場合、s.template <span style="color:#ff0000">subspan</span><2, 3>(); のように、</span>
<span class="w"> </span><span class="c1">// template限定子の指定が必要になることに注意</span>
<span class="w"> </span><span class="n"><a href="../span.html">std::span</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="o">></span><span class="w"> </span><span class="n">static_span</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">s</span><span class="p">.</span><span class="n"><span style="color:#ff0000">subspan</span></span><span class="o"><</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="o">></span><span class="p">();</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">static_span</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// (2) : 引数として開始位置と要素数を指定して、要素を取得する</span>
<span class="w"> </span><span class="n"><a href="../span.html">std::span</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../dynamic_extent.html">std::dynamic_extent</a></span><span class="o">></span><span class="w"> </span><span class="n">dynamic_span</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">s</span><span class="p">.</span><span class="n"><span style="color:#ff0000">subspan</span></span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">);</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">dynamic_span</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>3
4
5
3
4
5
</code></pre></p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++20</li>
</ul>
<h3>処理系</h3>
<ul>
<li><a href="../../../implementation.html#clang">Clang</a>: 9.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../../implementation.html#gcc">GCC</a>: ??</li>
<li><a href="../../../implementation.html#visual_cpp">Visual C++</a>: ??</li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1872r0.pdf" target="_blank">P1872R0 <code>span</code> should have <code>size_type</code>, not <code>index_type</code></a></li>
<li><a href="https://wg21.cmeerw.net/lwg/issue3103" target="_blank">LWG Issue 3103. Errors in taking subview of <code>span</code> should be ill-formed where possible</a></li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3471r4.html" target="_blank">P3471R4 Standard library hardening</a></li>
<li><a href="https://cplusplus.github.io/LWG/issue4293" target="_blank">LWG Issue 4293. <code>span::subspan</code>/<code>first</code>/<code>last</code> chooses wrong constructor when <code>T</code> is const-qualified <code>bool</code></a><ul>
<li>C++26で、効果が波カッコ初期化<code>R{...}</code>から丸カッコ構築<code>R(...)</code>へ変更された。要素型が<code>bool</code>（<code>const bool</code>など）のとき、リスト初期化が意図しないコンストラクタ（<code>initializer_list</code>をとるもの）を選んでしまう問題を防ぐもの</li>
</ul>
</li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3878r1.html" target="_blank">P3878R1 Standard library hardening should not use the 'observe' semantic</a></li>
</ul></div>
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