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<div class="identifier-type">function</div><div class="header"><compare></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">compare_weak_order_fallback</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="kr">inline</span><span class="w"> </span><span class="k">namespace</span><span class="w"> </span><span class="cm">/*unspecified*/</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kr">inline</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="cm">/*unspecified*/</span><span class="w"> </span><span class="n">compare_weak_order_fallback</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="cm">/*unspecified*/</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p><code>compare_weak_order_fallback</code>は2つの引数を受け取り、それらを弱順序の上で比較する関数オブジェクトである。<br />
<code><a href="weak_order.html">weak_order</a></code>が使用できない場合でも、引数型が<code>< ==</code>両演算子を使用可能であればそれを用いて比較を行う。</p>
<h2>効果</h2>
<p><code>compare_weak_order_fallback(a, b)</code>のように呼び出された時、以下のいずれかと等価(上から順に考慮される)</p>
<ol>
<li>
<p><code><a href="../type_traits/decay.html">decay</a></code>を通した<code>a, b</code>の型が異なる場合、呼び出しは<a class="cpprefjp-defined-word" data-desc="プログラムが適格でないこと。コンパイルエラーなどになる" href="../../implementation-compliance.html#dfn-ill-formed">不適格</a>(コンパイルエラー)</p>
</li>
<li>
<p><code><a href="weak_order.html">weak_order</a>(a, b)</code>が呼び出し可能ならば<code>weak_order(a, b)</code></p>
</li>
<li>
<p><code>a == b</code>、<code>a < b</code>の両方の演算子が使用可能でありその<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>型が<code>bool</code>へ変換可能ならば、以下の式
<br />
<div class="codehilite"><pre><span></span><code><span class="n">a</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">weak_ordering</span><span class="o">::</span><span class="n">equivalent</span><span class="w"> </span><span class="o">:</span>
<span class="n">a</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">weak_ordering</span><span class="o">::</span><span class="n">less</span><span class="w"> </span><span class="o">:</span>
<span class="w"> </span><span class="n">weak_ordering</span><span class="o">::</span><span class="n">greater</span>
</code></pre></div>
</p>
</li>
<li>
<p>それ以外の場合、呼び出しは<a class="cpprefjp-defined-word" data-desc="プログラムが適格でないこと。コンパイルエラーなどになる" href="../../implementation-compliance.html#dfn-ill-formed">不適格</a>。</p>
</li>
</ol>
<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>
<p>呼び出しが<a class="cpprefjp-defined-word" data-desc="プログラムが文法規則・診断対象の意味規則・単一定義規則を満たすこと" href="../../implementation-compliance.html#dfn-well-formed">適格</a>ならば、比較結果を表す<code><a href="weak_ordering.html">weak_ordering</a></code>の値。</p>
<h2><a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a></h2>
<p>上記「効果」節のそれぞれのケース毎に</p>
<ol>
<li>--</li>
<li>呼び出される<code>weak_order(a, b)</code>が<a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a>を送出するかに従う。</li>
<li>呼び出される<code>a < b</code>および<code>a == b</code>が<a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a>を送出するかに従う。</li>
</ol>
<h2>定数式に評価される条件</h2>
<p>上記「効果」節のそれぞれのケース毎に</p>
<ol>
<li>--</li>
<li>呼び出される<code>weak_order(a, b)</code>が定数評価可能であるかに従う。</li>
<li>呼び出される<code>a < b</code>および<code>a == b</code>が定数評価可能であるかに従う。</li>
</ol>
<h2>カスタマイゼーションポイント</h2>
<p>上記「効果」節2,3のケースでは、<a class="cpprefjp-defined-word" data-desc="プログラム定義。ユーザー(プログラマ)によって定義されること(標準ライブラリで定義されるものを除く)">ユーザー定義</a>の<code>< ==</code>演算子を定義、もしくは<code>weak_order()</code>へアダプトしておくことによって実行される比較をカスタマイズすることができる。</p>
<ol>
<li>--</li>
<li>引数<code>a, b</code>の型<code>T</code>を<code><a href="weak_order.html">weak_order</a></code>にアダプトしておく。</li>
<li>引数<code>a, b</code>の型<code>T</code>に対して、使用可能な<code>< ==</code>演算子を定義しておく。</li>
</ol>
<p>ただし、どのケースにおいてもその<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>型は<code><a href="weak_ordering.html">weak_ordering</a></code>に変換可能でなければならない。</p>
<h2>例</h2>
<p><div class="yata" id="8bc8c0ce8bf0d25a9f853ceea07c5d97b9d1388a"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../compare.html"><compare></a></span>
<span class="cp">#include <a href="../limits.html"><limits></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">legacy</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mf">0.0</span><span class="p">;</span>
<span class="w"> </span><span class="k">friend</span><span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="k">operator</span><span class="o">==</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">legacy</span><span class="o">&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">legacy</span><span class="o">&</span><span class="w"> </span><span class="n">rhs</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">lhs</span><span class="p">.</span><span class="n">v</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">rhs</span><span class="p">.</span><span class="n">v</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">friend</span><span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="k">operator</span><span class="o"><</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">legacy</span><span class="o">&</span><span class="w"> </span><span class="n">lhs</span><span class="p">,</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">legacy</span><span class="o">&</span><span class="w"> </span><span class="n">rhs</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">lhs</span><span class="p">.</span><span class="n">v</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">rhs</span><span class="p">.</span><span class="n">v</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</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="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="../ios/boolalpha.html">std::boolalpha</a></span><span class="p">;</span>
<span class="w"> </span><span class="n">legacy</span><span class="w"> </span><span class="n">l1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="o">+</span><span class="mf">0.0</span><span class="p">},</span><span class="w"> </span><span class="n">l2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mf">-0.0</span><span class="p">},</span><span class="w"> </span><span class="n">l3</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="o">-</span><span class="n"><a href="../limits/numeric_limits.html">std::numeric_limits</a></span><span class="o"><</span><span class="kt">double</span><span class="o">>::</span><span class="n">quiet_NaN</span><span class="p">()},</span><span class="w"> </span><span class="n">l4</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="n"><a href="../limits/numeric_limits.html">std::numeric_limits</a></span><span class="o"><</span><span class="kt">double</span><span class="o">>::</span><span class="n">quiet_NaN</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="p">(</span><span class="n">std</span><span class="o">::</span><span class="n"><span style="color:#ff0000">compare_weak_order_fallback</span></span><span class="p">(</span><span class="n">l1</span><span class="p">,</span><span class="w"> </span><span class="n">l2</span><span class="p">)</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">0</span><span class="p">)</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="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n"><span style="color:#ff0000">compare_weak_order_fallback</span></span><span class="p">(</span><span class="n">l1</span><span class="p">,</span><span class="w"> </span><span class="n">l2</span><span class="p">)</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">)</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="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="k">auto</span><span class="w"> </span><span class="n">comp1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n"><span style="color:#ff0000">compare_weak_order_fallback</span></span><span class="p">(</span><span class="n">l1</span><span class="p">,</span><span class="w"> </span><span class="n">l4</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="p">(</span><span class="n">comp1</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">)</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="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="p">(</span><span class="n">comp1</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="mi">0</span><span class="p">)</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="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="p">(</span><span class="n">comp1</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">0</span><span class="p">)</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="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="p">(</span><span class="n">comp1</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="mi">0</span><span class="p">)</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">//比較不能がgreaterと判定される</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="k">auto</span><span class="w"> </span><span class="n">comp2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n"><span style="color:#ff0000">compare_weak_order_fallback</span></span><span class="p">(</span><span class="n">l3</span><span class="p">,</span><span class="w"> </span><span class="n">l4</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="p">(</span><span class="n">comp2</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">)</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="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="p">(</span><span class="n">comp2</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="mi">0</span><span class="p">)</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="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="p">(</span><span class="n">comp2</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">0</span><span class="p">)</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="n"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="p">(</span><span class="n">comp2</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="mi">0</span><span class="p">)</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">//比較不能がgreaterと判定される</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>false
true
false
true
false
true
false
true
false
true
</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>: 14.0.6 <span aria-label="検証済" role="img" title="検証済">✅</span></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>: ??</li>
</ul>
<h2>関連項目</h2>
<ul>
<li><a href="../../lang/cpp20/consistent_comparison.html">C++20 <code><=></code>/<code>==</code>による比較演算子の自動定義</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://wg21.link/p0768" target="_blank">P0768R1 Library support for the spaceship (comparison) operator</a></li>
<li><a href="http://wg21.link/p1614" target="_blank">P1614R2 The Mothership has Landed (Adding <=> to the Library)</a></li>
</ul></div>
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