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71+ < header data-kunai-mdinfo ="{"meta": {"header": ["optional"], "namespace": ["std"], "class": ["optional"], "id-type": ["function"], "cpp": ["cpp26"]}, "sources": [{"id": "0e9914a5a89a8df5a4751094875ee0f90b43d490", "source": "#include <iostream>\n#include <optional>\n\nint main()\n{\n std::optional<int> p1 = 1;\n for (auto i = p1.begin(); i != p1.end(); ++i) {\n std::cout << *i; // \uff11\u5ea6\u901a\u308b\n }\n\n std::optional<int> p2 = std::null_opt;\n for (auto i = p2.begin(); i != p2.end(); ++i) {\n std::cout << *i; // \uff11\u5ea6\u3082\u901a\u3089\u306a\u3044\n }\n}\n"}], "page_id": ["reference", "optional", "optional", "begin"]} ">
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162+
163+ < span itemprop ="name "> optional</ span >
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174+
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202+ 最終更新日時(UTC):
203+ < span itemprop ="datePublished " content ="2025-11-12T11:34:03 ">
204+ 2025年11月12日 11時34分03秒
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210+ が更新
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216+ < span class ="fa fa-fw fa-clock-o fa-flip-horizontal "> </ span > 履歴
217+ </ a >
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219+ < span class ="fa fa-fw fa-pencil "> </ span > 編集
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223+ </ div >
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226+
227+ < div class ="identifier-type "> function</ div > < div class ="header "> <optional></ div > < h1 itemprop ="name "> < span class ="namespace " title ="namespace std "> std::</ span > < span class ="class " title ="class optional "> optional::</ span > < span class ="token "> begin</ span > < span class ="cpp cpp26 " title ="C++26で追加 "> (C++26)</ span > </ h1 >
228+ < div itemprop ="articleBody "> < p > < div class ="codehilite "> < pre > < span > </ span > < code > < span class ="k "> constexpr</ span > < span class ="n "> iterator</ span > < span class ="nf "> begin</ span > < span class ="p "> ()</ span > < span class ="k "> noexcept</ span > < span class ="p "> ;</ span > < span class ="c1 "> // (1)</ span >
229+ < span class ="k "> constexpr</ span > < span class ="n "> const_iterator</ span > < span class ="nf "> begin</ span > < span class ="p "> ()</ span > < span class ="k "> const</ span > < span class ="k "> noexcept</ span > < span class ="p "> ;</ span > < span class ="c1 "> // (2)</ span >
230+ </ code > </ pre > </ div >
231+ </ p >
232+ < h2 > 概要</ h2 >
233+ < p > < code > optional</ code > をrangeとした時の先頭要素を指すイテレータを取得する。</ p >
234+ < h2 > 効果</ h2 >
235+ < p > < code > < a href ="has_value.html "> has_value()</ a > == true</ code > なら、保持している有効値を指すイテレータを返す。またこの時、< code > < a href ="../../iterator/distance.html "> distance</ a > (begin(), < a href ="end.html "> end()</ a > ) == 1</ code > となる。</ p >
236+ < p > < code > < a href ="has_value.html "> has_value()</ a > != true</ code > なら、末尾要素の次を指すイテレータを返す。この時、< code > begin() == < a href ="end.html "> end()</ a > </ code > である。</ p >
237+ < h2 > 例</ h2 >
238+ < p > < div class ="yata " id ="0e9914a5a89a8df5a4751094875ee0f90b43d490 "> < div class ="codehilite "> < pre > < span > </ span > < code > < span class ="cp "> #include</ span > < span class ="cpf "> < a href ="../../iostream.html "> <iostream></ a > </ span > < span class ="cp "> </ span >
239+ < span class ="cp "> #include</ span > < span class ="cpf "> < a href ="../../optional.html "> <optional></ a > </ span > < span class ="cp "> </ span >
240+
241+ < span class ="kt "> int</ span > < span class ="nf "> main</ span > < span class ="p "> ()</ span >
242+ < span class ="p "> {</ span >
243+ < span class ="n "> < a href ="../optional.html "> std::optional</ a > </ span > < span class ="o "> <</ span > < span class ="kt "> int</ span > < span class ="o "> ></ span > < span class ="n "> p1</ span > < span class ="o "> =</ span > < span class ="mi "> 1</ span > < span class ="p "> ;</ span >
244+ < span class ="k "> for</ span > < span class ="p "> (</ span > < span class ="k "> auto</ span > < span class ="n "> i</ span > < span class ="o "> =</ span > < span class ="n "> p1</ span > < span class ="p "> .</ span > < span class ="n "> < span style ="color:#ff0000 "> begin()</ span > </ span > < span class ="p "> ;</ span > < span class ="n "> i</ span > < span class ="o "> !=</ span > < span class ="n "> p1</ span > < span class ="p "> .</ span > < span class ="n "> < a href ="end.html "> end()</ a > </ span > < span class ="p "> ;</ span > < span class ="o "> ++</ span > < span class ="n "> i</ span > < span class ="p "> )</ span > < span class ="p "> {</ span >
245+ < span class ="n "> < a href ="../../iostream/cout.html "> std::cout</ a > </ span > < span class ="o "> <<</ span > < span class ="o "> *</ span > < span class ="n "> i</ span > < span class ="p "> ;</ span > < span class ="c1 "> // １度通る</ span >
246+ < span class ="p "> }</ span >
247+
248+ < span class ="n "> < a href ="../optional.html "> std::optional</ a > </ span > < span class ="o "> <</ span > < span class ="kt "> int</ span > < span class ="o "> ></ span > < span class ="n "> p2</ span > < span class ="o "> =</ span > < span class ="n "> std</ span > < span class ="o "> ::</ span > < span class ="n "> null_opt</ span > < span class ="p "> ;</ span >
249+ < span class ="k "> for</ span > < span class ="p "> (</ span > < span class ="k "> auto</ span > < span class ="n "> i</ span > < span class ="o "> =</ span > < span class ="n "> p2</ span > < span class ="p "> .</ span > < span class ="n "> < span style ="color:#ff0000 "> begin()</ span > </ span > < span class ="p "> ;</ span > < span class ="n "> i</ span > < span class ="o "> !=</ span > < span class ="n "> p2</ span > < span class ="p "> .</ span > < span class ="n "> < a href ="end.html "> end()</ a > </ span > < span class ="p "> ;</ span > < span class ="o "> ++</ span > < span class ="n "> i</ span > < span class ="p "> )</ span > < span class ="p "> {</ span >
250+ < span class ="n "> < a href ="../../iostream/cout.html "> std::cout</ a > </ span > < span class ="o "> <<</ span > < span class ="o "> *</ span > < span class ="n "> i</ span > < span class ="p "> ;</ span > < span class ="c1 "> // １度も通らない</ span >
251+ < span class ="p "> }</ span >
252+ < span class ="p "> }</ span >
253+ </ code > </ pre > </ div >
254+ </ div > </ p >
255+ < h3 > 出力</ h3 >
256+ < p > < pre > < code > 1
257+ </ code > </ pre > </ p >
258+ < h2 > バージョン</ h2 >
259+ < h3 > 言語</ h3 >
260+ < ul >
261+ < li > C++26</ li >
262+ </ ul >
263+ < h3 > 処理系</ h3 >
264+ < ul >
265+ < li > < a href ="../../../implementation.html#clang "> Clang</ a > : ??</ li >
266+ < li > < a href ="../../../implementation.html#gcc "> GCC</ a > : ??</ li >
267+ < li > < a href ="../../../implementation.html#icc "> ICC</ a > : ??</ li >
268+ < li > < a href ="../../../implementation.html#visual_cpp "> Visual C++</ a > : ??</ li >
269+ </ ul >
270+ < h2 > 参照</ h2 >
271+ < ul >
272+ < li > < a href ="https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3168r2.html " target ="_blank "> P3168R2 Give std::optional Range Support</ a > </ li >
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274+
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