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<div class="identifier-type">concept</div><div class="header"><concepts></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">invocable</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">namespace</span><span class="w"> </span><span class="nn">std</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="w"> </span><span class="k">concept</span><span class="w"> </span><span class="nc"><a href="">invocable</a></span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">requires</span><span class="p">(</span><span class="n">F</span><span class="o">&&</span><span class="w"> </span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="n">Args</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../functional/invoke.html">invoke</a></span><span class="p">(</span><span class="n"><a href="../utility/forward.html">std::forward</a></span><span class="o"><</span><span class="n">F</span><span class="o">></span><span class="p">(</span><span class="n">f</span><span class="p">),</span><span class="w"> </span><span class="n"><a href="../utility/forward.html">std::forward</a></span><span class="o"><</span><span class="n">Args</span><span class="o">></span><span class="p">(</span><span class="n">args</span><span class="p">)...);</span>
<span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="w"> </span><span class="k">concept</span><span class="w"> </span><span class="nc"><a href="">regular_invocable</a></span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="">invocable</a></span><span class="o"><</span><span class="n">F</span><span class="p">,</span><span class="w"> </span><span class="n">Args</span><span class="p">...</span><span class="o">></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p><code>invocable</code>及び<code>regular_invocable</code>は、任意の関数呼び出し可能な型<code>F</code>が引数<code>Args...</code>によって関数呼び出し可能であることを表すコンセプトである。</p>
<p>加えて、<code>regular_invocable</code>はその呼び出しが等しさを保持し、なおかつ関数オブジェクト（<code>F</code>のオブジェクト）も引数（<code>Args</code>オブジェクト）も変更しないものである必要がある。</p>
<h2>等しさの保持</h2>
<p><code>invocable</code>コンセプトでは<code>F</code>の<code>Args...</code>による関数呼び出しが<a href="../concepts.html">等しさを保持</a>することを要求しない。従って、<code>invocable</code>コンセプトのモデルとなる型<code>F, Args...</code>は関数呼び出しに際して副作用があっても良く、その出力が内部状態や外部状態に依存していても構わない。</p>
<p>対して、<code>regular_invocable</code>コンセプトのモデルとなる<code>F</code>の<code>Args...</code>による関数呼び出しには<a href="../concepts.html">等しさを保持</a>することと引数を変更しないことが要求される。従って、<code>regular_invocable</code>コンセプトのモデルとなる型<code>F, Args...</code>は関数呼び出しに際して副作用を持ってはならず、出力は何かしらの状態に依存してはならない。ただし、このことは構文的に（コンパイル時に）チェックされるものではなく、純粋に意味論的な制約として要求・表明される。</p>
<p>例えば、<a href="../random.html">乱数・分布生成器</a>はその呼び出しに際して等しさを保持しない（内部に状態を保ち、出力はそれに依存する）ため、<code>regular_invocable</code>コンセプトのモデルにはならないが<code>invocable</code>コンセプトのモデルとなる。</p>
<h2>例</h2>
<h3>invocable</h3>
<p><div class="yata" id="81c3c2d779bd32dbbddd9ce4daf703e9fb36a189"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../concepts.html"><concepts></a></span>
<span class="cp">#include <a href="../random.html"><random></a></span>
<span class="k">template</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">F</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="k">requires</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::invocable</span></span><span class="o"><</span><span class="n">F</span><span class="p">,</span><span class="w"> </span><span class="n">Args</span><span class="p">...</span><span class="o">></span>
<span class="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="kt">char</span><span class="o">*</span><span class="w"> </span><span class="n">name</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">name</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" is <a href="">invocable</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="p">}</span>
<span class="k">template</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">F</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="kt">char</span><span class="o">*</span><span class="w"> </span><span class="n">name</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">name</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" is not <a href="">invocable</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="p">}</span>
<span class="kt">void</span><span class="w"> </span><span class="n">func</span><span class="p">(</span><span class="kt">int</span><span class="p">);</span>
<span class="k">auto</span><span class="w"> </span><span class="n">lambda</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[](</span><span class="k">auto</span><span class="w"> </span><span class="n">a</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">a</span><span class="p">;</span><span class="w"> </span><span class="p">};</span>
<span class="k">auto</span><span class="w"> </span><span class="n">mut_lambda</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[</span><span class="n">n</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">](</span><span class="k">auto</span><span class="w"> </span><span class="n">a</span><span class="p">)</span><span class="w"> </span><span class="k">mutable</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="o">++</span><span class="n">n</span><span class="p">;</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">a</span><span class="p">;</span><span class="w"> </span><span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc"><a href="">invocable</a></span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="k">operator</span><span class="p">()(</span><span class="n">T</span><span class="o">&&</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">t</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">not_invocable</span><span class="w"> </span><span class="p">{};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">func</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"func(int)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">lambda</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"lambda(int)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">lambda</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">*></span><span class="p">(</span><span class="s">"lambda(int*)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n"><a href="">invocable</a></span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"<a href="">invocable</a>(int)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n"><a href="">invocable</a></span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">***></span><span class="p">(</span><span class="s">"<a href="">invocable</a>(int***)"</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="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// 内部状態を保ち、等しさを保持しない呼び出し可能な型</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">mut_lambda</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"mut_lambda(int)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n"><a href="../random/mt19937.html">std::mt19937</a></span><span class="o">></span><span class="p">(</span><span class="s">"<a href="../random/mt19937.html">std::mt19937</a>()"</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="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">func</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">*></span><span class="p">(</span><span class="s">"func(int*)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n">not_invocable</span><span class="o">></span><span class="p">(</span><span class="s">"not_invocable()"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n">not_invocable</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"not_invocable(int)"</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>func(int) is <a href="">invocable</a>
lambda(int) is <a href="">invocable</a>
lambda(int*) is <a href="">invocable</a>
<a href="">invocable</a>(int) is <a href="">invocable</a>
<a href="">invocable</a>(int***) is <a href="">invocable</a>
mut_lambda(int) is <a href="">invocable</a>
<a href="../random/mt19937.html">std::mt19937</a>() is <a href="">invocable</a>
func(int*) is not <a href="">invocable</a>
not_invocable() is not <a href="">invocable</a>
not_invocable(int) is not <a href="">invocable</a>
</code></pre></p>
<h3>regular_invocable</h3>
<p><div class="yata" id="e74bca145b058a018b4ec76a5063343e4291ce73"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../concepts.html"><concepts></a></span>
<span class="cp">#include <a href="../random.html"><random></a></span>
<span class="k">template</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">F</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="k">requires</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::regular_invocable</span></span><span class="o"><</span><span class="n">F</span><span class="p">,</span><span class="w"> </span><span class="n">Args</span><span class="p">...</span><span class="o">></span>
<span class="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="kt">char</span><span class="o">*</span><span class="w"> </span><span class="n">name</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">name</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" is <a href="">regular_invocable</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="p">}</span>
<span class="k">template</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">F</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="kt">char</span><span class="o">*</span><span class="w"> </span><span class="n">name</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">name</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" is not <a href="">regular_invocable</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="p">}</span>
<span class="kt">void</span><span class="w"> </span><span class="n">func</span><span class="p">(</span><span class="kt">int</span><span class="p">);</span>
<span class="k">auto</span><span class="w"> </span><span class="n">lambda</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[](</span><span class="k">auto</span><span class="w"> </span><span class="n">a</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">a</span><span class="p">;</span><span class="w"> </span><span class="p">};</span>
<span class="k">auto</span><span class="w"> </span><span class="n">mut_lambda</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[</span><span class="n">n</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">](</span><span class="k">auto</span><span class="w"> </span><span class="n">a</span><span class="p">)</span><span class="w"> </span><span class="k">mutable</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="o">++</span><span class="n">n</span><span class="p">;</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">a</span><span class="p">;</span><span class="w"> </span><span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc"><a href="">invocable</a></span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="k">operator</span><span class="p">()(</span><span class="n">T</span><span class="o">&&</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">t</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">not_invocable</span><span class="w"> </span><span class="p">{};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">func</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"func(int)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">lambda</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"lambda(int)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">lambda</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">*></span><span class="p">(</span><span class="s">"lambda(int*)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n"><a href="">invocable</a></span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"<a href="">invocable</a>(int)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n"><a href="">invocable</a></span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">***></span><span class="p">(</span><span class="s">"<a href="">invocable</a>(int***)"</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="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// 内部状態を保ち、等しさを保持しない呼び出し可能な型</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">mut_lambda</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"mut_lambda(int)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n"><a href="../random/mt19937.html">std::mt19937</a></span><span class="o">></span><span class="p">(</span><span class="s">"<a href="../random/mt19937.html">std::mt19937</a>()"</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// これらの型は <span style="color:#ff0000">std::regular_invocable</span> コンセプトのモデルではないが</span>
<span class="w"> </span><span class="c1">// C++構文上では std::<a href="">invocable</a> との差異を区別しない／できないため</span>
<span class="w"> </span><span class="c1">// それぞれ「XXX is <a href="">regular_invocable</a>」と出力される。</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="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">func</span><span class="p">),</span><span class="w"> </span><span class="kt">int</span><span class="o">*></span><span class="p">(</span><span class="s">"func(int*)"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n">not_invocable</span><span class="o">></span><span class="p">(</span><span class="s">"not_invocable()"</span><span class="p">);</span>
<span class="w"> </span><span class="n">f</span><span class="o"><</span><span class="n">not_invocable</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="s">"not_invocable(int)"</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>func(int) is <a href="">regular_invocable</a>
lambda(int) is <a href="">regular_invocable</a>
lambda(int*) is <a href="">regular_invocable</a>
<a href="">invocable</a>(int) is <a href="">regular_invocable</a>
<a href="">invocable</a>(int***) is <a href="">regular_invocable</a>
mut_lambda(int) is <a href="">regular_invocable</a>
<a href="../random/mt19937.html">std::mt19937</a>() is <a href="">regular_invocable</a>
func(int*) is not <a href="">regular_invocable</a>
not_invocable() is not <a href="">regular_invocable</a>
not_invocable(int) is not <a href="">regular_invocable</a>
</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>: ??</li>
<li><a href="../../implementation.html#gcc">GCC</a>: 10.1 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: 2019 Update 3 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
</ul>
<h2>関連項目</h2>
<ul>
<li><a href="../../lang/cpp20/concepts.html">C++20 コンセプト</a></li>
<li><a href="../functional/invoke.html">invoke</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0898r3.pdf" target="_blank">P0898R3 Standard Library Concepts</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1754r1.pdf" target="_blank">P1754R1 Rename concepts to standard_case for C++20, while we still can</a></li>
</ul></div>
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