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<div class="identifier-type">function</div><div class="header"><cmath></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">isless</span><span class="cpp cpp11" title="C++11で追加">(C++11)</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="kt">bool</span><span class="w"> </span><span class="nf">isless</span><span class="p">(</span><span class="kt">float</span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="kt">float</span><span class="w"> </span><span class="n">y</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++11からC++20まで</span>
<span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="nf">isless</span><span class="p">(</span><span class="kt">double</span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">y</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++11からC++20まで</span>
<span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="nf">isless</span><span class="p">(</span><span class="kt">long</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">y</span><span class="p">);</span><span class="w"> </span><span class="c1">// (3) C++11からC++20まで</span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="kt">bool</span>
<span class="w"> </span><span class="nf">isless</span><span class="p">(</span><span class="n"><i>floating-point-type</i></span><span class="w"> </span><span class="n">x</span><span class="p">,</span>
<span class="w"> </span><span class="n"><i>floating-point-type</i></span><span class="w"> </span><span class="n">y</span><span class="p">);</span><span class="w"> </span><span class="c1">// (4) C++23</span>
<span class="w"> </span><span class="kt">bool</span>
<span class="w"> </span><span class="nf">isless</span><span class="p">(</span><span class="n"><i>Integral</i></span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n"><i>Integral</i></span><span class="w"> </span><span class="n">y</span><span class="p">);</span><span class="w"> </span><span class="c1">// (5) C++11</span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="kt">bool</span>
<span class="w"> </span><span class="nf">isless</span><span class="p">(</span><span class="n"><i>Integral</i></span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n"><i>Integral</i></span><span class="w"> </span><span class="n">y</span><span class="p">);</span><span class="w"> </span><span class="c1">// (5) C++23</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>浮動小数点数について、左辺が右辺より小さいかを判定する。</p>
<ul>
<li>(1) : <code>float</code>に対する<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a></li>
<li>(2) : <code>double</code>に対する<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a></li>
<li>(3) : <code>long double</code>に対する<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a></li>
<li>(4) : 浮動小数点数型に対する<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a></li>
<li>(5) : 整数型に対する<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a> (<code>double</code>にキャストして計算される)</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>
<p><code>x < y</code>と等価の演算によって、<code>x</code>が<code>y</code>より小さければ<code>true</code>、そうでなければ<code>false</code>を返す。</p>
<p><code>x < y</code>と違って、この関数は<code>x</code>と<code>y</code>が順序付けされていない場合に、<code><a href="../cfenv/fe_invalid.html">FE_INVALID</a></code>（無効演算<a class="cpprefjp-defined-word" data-desc="floating-point exception。ゼロ除算やオーバーフロー等の算術的な問題が発生した際に、浮動小数点環境に設定される状態フラグのこと。C++言語機能の例外機構（try-catch）とは異なる。" href="../cfenv.html">浮動小数点例外</a>）は発生しない。</p>
<h2>備考</h2>
<ul>
<li>C標準ライブラリでは本関数は関数マクロとして定義されるが、C++標準ライブラリでは関数として定義される</li>
<li>C標準ライブラリでは本関数は<code>int</code>型を<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>とするが、C++標準ライブラリでは<code>bool</code>型を<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>とする</li>
<li>C++23では、(1), (2), (3)が(4)に統合され、<a class="cpprefjp-defined-word" data-desc="<stdfloat>で定義される内部表現の規定された浮動小数点数型" href="../stdfloat.html">拡張浮動小数点数型</a>を含む浮動小数点数型への<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a>として定義された</li>
</ul>
<h2>例</h2>
<p><div class="yata" id="133f9db4a1fdcd7b4e27955b545f7bc8a1dde1a5"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../cmath.html"><cmath></a></span>
<span class="cp">#include <a href="../limits.html"><limits></a></span>
<span class="kt">void</span><span class="w"> </span><span class="nf">test</span><span class="p">(</span><span class="kt">double</span><span class="w"> </span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">y</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"><a href="../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"isless("</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="s">", "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">y</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">") = "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::isless</span></span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="w"> </span><span class="n">y</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="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">test</span><span class="p">(</span><span class="mf">1.0</span><span class="p">,</span><span class="w"> </span><span class="mf">2.0</span><span class="p">);</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="mf">2.0</span><span class="p">,</span><span class="w"> </span><span class="mf">1.0</span><span class="p">);</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="mf">2.0</span><span class="p">,</span><span class="w"> </span><span class="mf">2.0</span><span class="p">);</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="mf">-0.0</span><span class="p">,</span><span class="w"> </span><span class="mf">0.0</span><span class="p">);</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">inf</span><span class="w"> </span><span class="o">=</span><span class="w"> </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"><a href="../limits/numeric_limits/infinity.html">infinity()</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="kt">double</span><span class="w"> </span><span class="n">nan</span><span class="w"> </span><span class="o">=</span><span class="w"> </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"><a href="../limits/numeric_limits/quiet_nan.html">quiet_NaN()</a></span><span class="p">;</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="mf">1.0</span><span class="p">,</span><span class="w"> </span><span class="n">inf</span><span class="p">);</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="n">inf</span><span class="p">,</span><span class="w"> </span><span class="mf">1.0</span><span class="p">);</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="n">inf</span><span class="p">,</span><span class="w"> </span><span class="n">inf</span><span class="p">);</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="mf">1.0</span><span class="p">,</span><span class="w"> </span><span class="n">nan</span><span class="p">);</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="n">nan</span><span class="p">,</span><span class="w"> </span><span class="mf">1.0</span><span class="p">);</span>
<span class="w"> </span><span class="n">test</span><span class="p">(</span><span class="n">nan</span><span class="p">,</span><span class="w"> </span><span class="n">nan</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力例</h3>
<p><pre><code>isless(1, 2) = true
isless(2, 1) = false
isless(2, 2) = false
isless(-0, 0) = false
isless(1, inf) = true
isless(inf, 1) = false
isless(inf, inf) = false
isless(1, nan) = false
isless(nan, 1) = false
isless(nan, nan) = false
</code></pre></p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++11</li>
</ul>
<h3>処理系</h3>
<ul>
<li><a href="../../implementation.html#clang">Clang</a>: 3.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#gcc">GCC</a>: 4.3 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#icc">ICC</a>: ??</li>
<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: ??</li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0533r9.pdf" target="_blank">P0533R9 constexpr for <code><cmath></code> and <code><cstdlib></code></a><ul>
<li>C++23での、一部関数の<code>constexpr</code>対応</li>
</ul>
</li>
<li><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1467r9.html" target="_blank">P1467R9 Extended floating-point types and standard names</a><ul>
<li>C++23で導入された<a class="cpprefjp-defined-word" data-desc="<stdfloat>で定義される内部表現の規定された浮動小数点数型" href="../stdfloat.html">拡張浮動小数点数型</a>への対応として、<code>float</code>、<code>double</code>、<code>long double</code>の<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a>を<code>floating-point-type</code>の<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a>に統合し、<a class="cpprefjp-defined-word" data-desc="<stdfloat>で定義される内部表現の規定された浮動小数点数型" href="../stdfloat.html">拡張浮動小数点数型</a>も扱えるようにした</li>
</ul>
</li>
</ul></div>
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