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<div class="identifier-type">function</div><div class="header"><atomic></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="class" title="class atomic">atomic::</span><span class="token">fetch_sub</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="n">T</span>
<span class="w"> </span><span class="nf">fetch_sub</span><span class="p">(</span><span class="n">difference_type</span><span class="w"> </span><span class="n">operand</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../memory_order.html">memory_order</a></span><span class="w"> </span><span class="n">order</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../memory_order.html">memory_order_seq_cst</a></span>
<span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="k">volatile</span><span class="w"> </span><span class="k">noexcept</span><span class="p">;</span><span class="w"> </span><span class="c1">// (1) C++11</span>
<span class="n">T</span>
<span class="w"> </span><span class="nf">fetch_sub</span><span class="p">(</span><span class="n">difference_type</span><span class="w"> </span><span class="n">operand</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../memory_order.html">memory_order</a></span><span class="w"> </span><span class="n">order</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../memory_order.html">memory_order_seq_cst</a></span>
<span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="k">noexcept</span><span class="p">;</span><span class="w"> </span><span class="c1">// (2) C++11</span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">T</span>
<span class="w"> </span><span class="nf">fetch_sub</span><span class="p">(</span><span class="n">difference_type</span><span class="w"> </span><span class="n">operand</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../memory_order.html">memory_order</a></span><span class="w"> </span><span class="n">order</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../memory_order.html">memory_order_seq_cst</a></span>
<span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="k">noexcept</span><span class="p">;</span><span class="w"> </span><span class="c1">// (2) C++26</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>減算を行う</p>
<h2>要件</h2>
<ul>
<li>(1), (2) :<ul>
<li>C++17 : <code>std::atomic<T*></code>の場合、型<code>T</code>がオブジェクト型であること。型<code>T</code>が<code>void*</code>や関数ポインタであってはならない</li>
</ul>
</li>
<li>(1) :<ul>
<li>C++20 : <code>atomic<T>::is_always_lock_free</code>が<code>true</code>であること</li>
</ul>
</li>
</ul>
<h2>効果</h2>
<p><code>order</code>で指定されたメモリオーダーにしたがって、現在の値に<code>operand</code>を減算した値でアトミックに置き換える</p>
<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>変更前の値が返される</p>
<h2>備考</h2>
<ul>
<li>この関数は、<code>atomic</code>クラスの整数型、浮動小数点数型 (C++20)、ポインタに対する特殊化で定義される</li>
<li>整数型<ul>
<li>符号付き整数型に対しては、符号なし整数型に変換されたかのようにしたあと演算が行われ、結果は符号付き整数型になる。<a class="cpprefjp-defined-word" data-desc="未定義の動作。処理系は予期せぬ動作をする可能性がある。要するに動作保証対象外。undefined behavior (UB)。" href="../../../implementation-compliance.html#dfn-undefined-behavior">未定義動作</a>はない</li>
</ul>
</li>
<li>浮動小数点数型 (C++20)<ul>
<li>演算結果が、その型で表現できない値であった場合、結果は<a class="cpprefjp-defined-word" data-desc="未規定の動作。事前の断りなく処理系によって異なる動作をする" href="../../../implementation-compliance.html#dfn-unspecified-behavior">未規定</a>値になる。ただしその操作によって<a class="cpprefjp-defined-word" data-desc="未定義の動作。処理系は予期せぬ動作をする可能性がある。要するに動作保証対象外。undefined behavior (UB)。" href="../../../implementation-compliance.html#dfn-undefined-behavior">未定義動作</a>は起こらない</li>
<li>浮動小数点数型に対する操作は<code><a href="../../limits/numeric_limits.html">std::numeric_limits</a><floating-point></code>トレイトに準拠する</li>
<li>浮動小数点数型に対するアトミック操作の浮動小数点環境は、呼び出しスレッドの浮動小数点環境とは異なる可能性がある</li>
</ul>
</li>
<li>ポインタ型<ul>
<li>結果として<a class="cpprefjp-defined-word" data-desc="通常は未定義動作となるポインタの加減算が atomic 操作として行われた場合の結果を指す規格中の表記「undefined address」を指す。その意味付けは不明確なので LWG issue 3906. Undefined address is undefined が提起されている。atomic操作は未定義動作を含まないことを意図して設計されているため未定義アドレスは未定義動作ではないと考えられる" href="https://cplusplus.github.io/LWG/issue3906" target="_blank">未定義アドレス</a>になる場合があるが、それ以外の<a class="cpprefjp-defined-word" data-desc="未定義の動作。処理系は予期せぬ動作をする可能性がある。要するに動作保証対象外。undefined behavior (UB)。" href="../../../implementation-compliance.html#dfn-undefined-behavior">未定義動作</a>はない</li>
</ul>
</li>
</ul>
<h2>例</h2>
<h3>整数の例 (C++11)</h3>
<p><div class="yata" id="947d45717f32d550727aecf5abaa02b2280d8fa8"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../atomic.html"><atomic></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="../atomic.html">std::atomic</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">x</span><span class="p">(</span><span class="mi">3</span><span class="p">);</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">before</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">x</span><span class="p">.</span><span class="n"><span style="color:#ff0000">fetch_sub</span></span><span class="p">(</span><span class="mi">2</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">before</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="n"><a href="load.html">x.load()</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>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>3
1
</code></pre></p>
<h4>浮動小数点数の例 (C++20)</h4>
<p><div class="yata" id="684357392cbf16bfe633b05ec67eb3a626759b99"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../atomic.html"><atomic></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="../atomic.html">std::atomic</a></span><span class="o"><</span><span class="kt">float</span><span class="o">></span><span class="w"> </span><span class="n">x</span><span class="p">{</span><span class="mf">3.14f</span><span class="p">};</span>
<span class="w"> </span><span class="kt">float</span><span class="w"> </span><span class="n">before</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">x</span><span class="p">.</span><span class="n"><span style="color:#ff0000">fetch_sub</span></span><span class="p">(</span><span class="mf">1.25f</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">before</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="n"><a href="load.html">x.load()</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>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>3.14
1.89
</code></pre></p>
<h3>複数スレッドから減算する例 (C++11)</h3>
<p><div class="yata" id="b2eb4ace533392da8becb4c671e11338faf5aa44"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../atomic.html"><atomic></a></span>
<span class="cp">#include <a href="../../thread.html"><thread></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="../atomic.html">std::atomic</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">x</span><span class="p">{</span><span class="mi">10</span><span class="p">};</span>
<span class="w"> </span><span class="c1">// 複数スレッドで減算を呼んでも、</span>
<span class="w"> </span><span class="c1">// 最終的に全てのスレッドでの減算が処理された値になる</span>
<span class="w"> </span><span class="n"><a href="../../thread/thread.html">std::thread</a></span><span class="w"> </span><span class="n">t1</span><span class="w"> </span><span class="p">{[</span><span class="o">&</span><span class="n">x</span><span class="p">]</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">x</span><span class="p">.</span><span class="n"><span style="color:#ff0000">fetch_sub</span></span><span class="p">(</span><span class="mi">1</span><span class="p">);</span>
<span class="w"> </span><span class="p">}};</span>
<span class="w"> </span><span class="n"><a href="../../thread/thread.html">std::thread</a></span><span class="w"> </span><span class="n">t2</span><span class="w"> </span><span class="p">{[</span><span class="o">&</span><span class="n">x</span><span class="p">]</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">x</span><span class="p">.</span><span class="n"><span style="color:#ff0000">fetch_sub</span></span><span class="p">(</span><span class="mi">2</span><span class="p">);</span>
<span class="w"> </span><span class="p">}};</span>
<span class="w"> </span><span class="n"><a href="../../thread/thread/join.html">t1.join()</a></span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../thread/thread/join.html">t2.join()</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="n"><a href="load.html">x.load()</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>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>7
</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>: 3.2 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../../implementation.html#gcc">GCC</a>: 4.7.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../../implementation.html#visual_cpp">Visual C++</a>: 2012 <span aria-label="検証済" role="img" title="検証済">✅</span>, 2013 <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/deprecating_volatile.html">C++20 ほとんどの<code>volatile</code>を非推奨化</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0558r1.pdf" target="_blank">P0558R1 Resolving <code>atomic<T></code> named base class inconsistencies</a><ul>
<li>C++17での、オブジェクト型であることの制約追加</li>
</ul>
</li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0020r6.html" target="_blank">P0020R6 Floating Point Atomic</a><ul>
<li>C++20での、浮動小数点数版の追加</li>
</ul>
</li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1831r1.html" target="_blank">P1831R1 Deprecating <code>volatile</code>: library</a><ul>
<li>C++20での、<code>volatile</code>版への制約追加</li>
</ul>
</li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3309r3.html" target="_blank">P3309R3 <code>constexpr atomic</code> and <code>atomic_ref</code></a><ul>
<li>C++26で<code>constexpr</code>に対応した</li>
</ul>
</li>
</ul></div>
</div>
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