156 lines
5.2 KiB
Markdown
156 lines
5.2 KiB
Markdown
[variant.get]
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# 22 General utilities library [[utilities]](./#utilities)
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## 22.6 Variants [[variant]](variant#get)
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### 22.6.5 Value access [variant.get]
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[ð](#lib:holds_alternative)
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`template<class T, class... Types>
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constexpr bool holds_alternative(const variant<Types...>& v) noexcept;
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`
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6520)
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*Mandates*: The type T occurs exactly once in Types[.](#1.sentence-1)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6524)
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*Returns*: true if index() is equal to the zero-based index of T in Types[.](#2.sentence-1)
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[ð](#itemdecl:2)
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`template<size_t I, class... Types>
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constexpr variant_alternative_t<I, variant<Types...>>&
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GET(variant<Types...>& v); // exposition only
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template<size_t I, class... Types>
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constexpr variant_alternative_t<I, variant<Types...>>&&
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GET(variant<Types...>&& v); // exposition only
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template<size_t I, class... Types>
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constexpr const variant_alternative_t<I, variant<Types...>>&
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GET(const variant<Types...>& v); // exposition only
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template<size_t I, class... Types>
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constexpr const variant_alternative_t<I, variant<Types...>>&&
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GET(const variant<Types...>&& v); // exposition only
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`
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6545)
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*Mandates*: I < sizeof...(Types)[.](#3.sentence-1)
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6549)
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*Preconditions*: v.index() is I[.](#4.sentence-1)
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6553)
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*Returns*: A reference to the object stored in the variant[.](#5.sentence-1)
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[ð](#lib:get,variant)
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`template<size_t I, class... Types>
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constexpr variant_alternative_t<I, variant<Types...>>& get(variant<Types...>& v);
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template<size_t I, class... Types>
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constexpr variant_alternative_t<I, variant<Types...>>&& get(variant<Types...>&& v);
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template<size_t I, class... Types>
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constexpr const variant_alternative_t<I, variant<Types...>>& get(const variant<Types...>& v);
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template<size_t I, class... Types>
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constexpr const variant_alternative_t<I, variant<Types...>>&& get(const variant<Types...>&& v);
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`
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6571)
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*Mandates*: I < sizeof...(Types)[.](#6.sentence-1)
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6575)
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*Effects*: If v.index() is I, returns a reference to the object stored in
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the variant[.](#7.sentence-1)
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Otherwise, throws an exception of type bad_variant_access[.](#7.sentence-2)
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[ð](#lib:get,variant_)
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`template<class T, class... Types> constexpr T& get(variant<Types...>& v);
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template<class T, class... Types> constexpr T&& get(variant<Types...>&& v);
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template<class T, class... Types> constexpr const T& get(const variant<Types...>& v);
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template<class T, class... Types> constexpr const T&& get(const variant<Types...>&& v);
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`
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6590)
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*Mandates*: The type T occurs exactly once in Types[.](#8.sentence-1)
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[9](#9)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6594)
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*Effects*: If v holds a value of type T, returns a reference to that value[.](#9.sentence-1)
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Otherwise, throws an exception of type bad_variant_access[.](#9.sentence-2)
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[ð](#lib:get_if)
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`template<size_t I, class... Types>
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constexpr add_pointer_t<variant_alternative_t<I, variant<Types...>>>
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get_if(variant<Types...>* v) noexcept;
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template<size_t I, class... Types>
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constexpr add_pointer_t<const variant_alternative_t<I, variant<Types...>>>
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get_if(const variant<Types...>* v) noexcept;
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`
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[10](#10)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6612)
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*Mandates*: I < sizeof...(Types)[.](#10.sentence-1)
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[11](#11)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6616)
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*Returns*: A pointer to the value stored in the variant, if v != nullptr and v->index() == I[.](#11.sentence-1)
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Otherwise, returns nullptr[.](#11.sentence-2)
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[ð](#lib:get_if_)
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`template<class T, class... Types>
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constexpr add_pointer_t<T>
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get_if(variant<Types...>* v) noexcept;
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template<class T, class... Types>
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constexpr add_pointer_t<const T>
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get_if(const variant<Types...>* v) noexcept;
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`
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[12](#12)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6634)
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*Mandates*: The type T occurs exactly once in Types[.](#12.sentence-1)
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[13](#13)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6638)
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*Effects*: Equivalent to: return get_if<i>(v); with i being the zero-based
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index of T in Types[.](#13.sentence-1)
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