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174+ < span itemprop ="name "> 条件付きで特殊メンバ関数をトリビアルに定義するように</ span >
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190+ 最終更新日時(UTC):
191+ < span itemprop ="datePublished " content ="2025-01-23T16:56:13 ">
192+ 2025年01月23日 16時56分13秒
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196+ < span itemprop ="name "> onihusube</ span >
197+ </ span >
198+ が更新
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201+
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204+ < span class ="fa fa-fw fa-clock-o fa-flip-horizontal "> </ span > 履歴
205+ </ a >
206+ < a class ="edit " target ="_blank " href ="https://github.com/cpprefjp/site/edit/master/lang/cpp20/conditionally_trivial_special_member_functions.md ">
207+ < span class ="fa fa-fw fa-pencil "> </ span > 編集
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214+
215+ < h1 itemprop ="name "> < span class ="token "> 条件付きで特殊メンバ関数をトリビアルに定義するように</ span > < span class ="cpp cpp20 " title ="C++20で追加 "> (C++20)</ span > </ h1 >
216+ < div itemprop ="articleBody "> < p > このページはC++20に採用された言語機能の変更を解説しています。</ p >
217+ < p > のちのC++規格でさらに変更される場合があるため< a href ="#relative-page "> 関連項目</ a > を参照してください。</ p >
218+ < h2 > 概要</ h2 >
219+ < p > クラスの特殊メンバ関数（コンストラクタ・代入演算子・デストラクタ）の< code > default</ code > 宣言をコンセプトによって制約することで、テンプレートパラメータの性質に応じて優先的に選択させることができる様になる。</ p >
220+ < p > < div class ="codehilite "> < pre > < span > </ span > < code > < span class ="k "> template</ span > < span class ="o "> <</ span > < span class ="k "> typename</ span > < span class ="n "> T</ span > < span class ="o "> ></ span >
221+ < span class ="k "> struct</ span > < span class ="n "> wrap</ span > < span class ="p "> {</ span >
222+
223+ < span class ="c1 "> // 1. 制約されたdefaultなコピーコンストラクタ</ span >
224+ < span class ="n "> wrap</ span > < span class ="p "> (</ span > < span class ="k "> const</ span > < span class ="n "> wrap</ span > < span class ="o "> &</ span > < span class ="p "> )</ span > < span class ="n "> requires</ span > < span class ="n "> std</ span > < span class ="o "> ::</ span > < span class ="n "> is_trivially_copy_constructible_v</ span > < span class ="o "> <</ span > < span class ="n "> T</ span > < span class ="o "> ></ span > < span class ="o "> =</ span > < span class ="k "> default</ span > < span class ="p "> ;</ span >
225+
226+ < span class ="c1 "> // 2. ユーザー定義コピーコンストラクタ</ span >
227+ < span class ="n "> wrap</ span > < span class ="p "> (</ span > < span class ="k "> const</ span > < span class ="n "> wrap</ span > < span class ="o "> &</ span > < span class ="p "> )</ span > < span class ="p "> {</ span >
228+ < span class ="p "> ...</ span >
229+ < span class ="p "> }</ span >
230+ < span class ="p "> };</ span >
231+ </ code > </ pre > </ div >
232+ </ p >
233+ < p > < code > T</ code > が< code > std::is_trivially_copy_constructible</ code > を満たす場合、1の宣言が選択され< code > wrap<T></ code > のコピーコンストラクタは< code > default</ code > 実装される（2のコンストラクタが選択されることは無い）。< code > T</ code > が< code > std::is_trivially_copy_constructible</ code > を満たさない場合、2の宣言が選択され< code > wrap<T></ code > のコピーコンストラクタは< 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 > される。</ p >
234+ < p > 特に、< code > T</ code > がトリビアルコピー可能である場合にこの仕組みによって1のコンストラクタが選択されれば、（他の条件を満たしているものとして）< code > wrap<T></ code > もまたトリビアルコピー可能となる。</ p >
235+ < h2 > 仕様</ h2 >
236+ < p > (執筆中)</ p >
237+ < h2 > 例</ h2 >
238+ < p > (執筆中)</ p >
239+ < p > < div class ="yata " id ="f13c43f217dd0de2a102f881250a84a7c361e710 "> < div class ="codehilite "> < pre > < span > </ span > < code > < span class ="c1 "> // (ここには、言語機能の使い方を解説するための、サンプルコードを記述します。)</ span >
240+ < span class ="c1 "> // (インクルードとmain()関数を含む、実行可能なサンプルコードを記述してください。そのようなコードブロックにはexampleタグを付けます。)</ span >
241+
242+ < span class ="cp "> #include</ span > < span class ="cpf "> < a href ="../../reference/iostream.html "> <iostream></ a > </ span > < span class ="cp "> </ span >
243+
244+ < span class ="kt "> int</ span > < span class ="nf "> main</ span > < span class ="p "> ()</ span >
245+ < span class ="p "> {</ span >
246+ < span class ="kt "> int</ span > < span class ="n "> < span style ="color:#ff0000 "> variable</ span > </ span > < span class ="o "> =</ span > < span class ="mi "> 0</ span > < span class ="p "> ;</ span >
247+ < span class ="n "> < a href ="../../reference/iostream/cout.html "> std::cout</ a > </ span > < span class ="o "> <<</ span > < span class ="n "> < span style ="color:#ff0000 "> variable</ span > </ span > < span class ="o "> <<</ span > < span class ="n "> < a href ="../../reference/ostream/endl.html "> std::endl</ a > </ span > < span class ="p "> ;</ span >
248+ < span class ="p "> }</ span >
249+ </ code > </ pre > </ div >
250+ </ div > </ p >
251+ < h3 > 出力</ h3 >
252+ < p > < pre > < code > 0
253+ </ code > </ pre > </ p >
254+ < h2 > この機能が必要になった背景・経緯</ h2 >
255+ < p > (執筆中)</ p >
256+ < h2 > 検討されたほかの選択肢</ h2 >
257+ < p > (執筆中)</ p >
258+ < h2 > ## < a href ="#relative-page " id ="relative-page "> 関連項目</ a > </ h2 >
259+ < ul >
260+ < li > < a href ="concepts.html "> コンセプト</ a > </ li >
261+ </ ul >
262+ < h2 > 参照</ h2 >
263+ < ul >
264+ < li > < a href ="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0848r0.html " target ="_blank "> P0848R0 Conditionally Trivial Special Member Functions</ a > </ li >
265+ < li > < a href ="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0848r3.html " target ="_blank "> P0848R3 Conditionally Trivial Special Member Functions</ a > </ li >
266+ </ ul > </ div >
267+
268+ </ div >
269+ </ div >
270+ </ div >
271+
272+
273+ < div id ="sidebar " class ="col-sm-3 col-sm-pull-9 ">
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275+
276+
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282+ < p > < small >
283+ 本サイトの情報は、
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285+ の下に提供されています。
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