118 lines
4.6 KiB
Markdown
118 lines
4.6 KiB
Markdown
[tuple.helper]
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# 22 General utilities library [[utilities]](./#utilities)
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## 22.4 Tuples [[tuple]](tuple#helper)
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### 22.4.7 Tuple helper classes [tuple.helper]
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[ð](#lib:tuple_size,in_general)
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`template<class T> struct tuple_size;
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`
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2761)
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Except where specified otherwise,
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all specializations of tuple_size meet the[*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements")) with a
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base characteristic of integral_constant<size_t, N> for some N[.](#1.sentence-1)
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[ð](#lib:tuple_size)
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`template<class... Types>
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struct tuple_size<tuple<Types...>> : integral_constant<size_t, sizeof...(Types)> { };
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`
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[ð](#lib:tuple_element)
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`template<size_t I, class... Types>
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struct tuple_element<I, tuple<Types...>> {
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using type = TI;
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};
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`
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2784)
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*Mandates*: I < sizeof...(Types)[.](#2.sentence-1)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2788)
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*Type*: TI is the
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type of the Ith element of Types,
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where indexing is zero-based[.](#3.sentence-1)
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[ð](#lib:tuple_size_)
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`template<class T> struct tuple_size<const T>;
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`
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[4](#4)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2801)
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Let TS denote tuple_size<T> of the cv-unqualified type T[.](#4.sentence-1)
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If the expression TS::value is well-formed
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when treated as an [unevaluated operand](expr.context#def:unevaluated_operand "7.2.3 Context dependence [expr.context]"), then
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each specialization of the template meets the [*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))
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with a base characteristic ofintegral_constant<size_t, TS::value>
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Otherwise, it has no member value[.](#4.sentence-3)
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[5](#5)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2812)
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Access checking is performed as if in a context
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unrelated to TS and T[.](#5.sentence-1)
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Only the validity of the immediate context of the expression is considered[.](#5.sentence-2)
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[*Note [1](#note-1)*:
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The compilation of the expression can result in side effects
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such as the instantiation of class template specializations and
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function template specializations, the generation of implicitly-defined functions, and so on[.](#5.sentence-3)
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Such side effects are not in the âimmediate contextâ and
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can result in the program being ill-formed[.](#5.sentence-4)
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â *end note*]
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[6](#6)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2824)
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In addition to being available via inclusion of the [<tuple>](tuple.syn#header:%3ctuple%3e "22.4.2 Header <tuple> synopsis [tuple.syn]") header,
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the template is available
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when any of the headers[<array>](array.syn#header:%3carray%3e "23.3.2 Header <array> synopsis [array.syn]"),[<ranges>](ranges.syn#header:%3cranges%3e "25.2 Header <ranges> synopsis [ranges.syn]"), or[<utility>](utility.syn#header:%3cutility%3e "22.2.1 Header <utility> synopsis [utility.syn]") are included[.](#6.sentence-1)
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[ð](#lib:tuple_element_)
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`template<size_t I, class T> struct tuple_element<I, const T>;
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`
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[7](#7)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2840)
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Let TE denote tuple_element_t<I, T> of the cv-unqualified type T[.](#7.sentence-1)
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Then
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each specialization of the template meets the [*Cpp17TransformationTrait*](meta.rqmts#:Cpp17TransformationTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))
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with a member typedef type that names the type add_const_t<TE>[.](#7.sentence-2)
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[8](#8)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2845)
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In addition to being available via inclusion of the [<tuple>](tuple.syn#header:%3ctuple%3e "22.4.2 Header <tuple> synopsis [tuple.syn]") header,
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the template is available
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when any of the headers[<array>](array.syn#header:%3carray%3e "23.3.2 Header <array> synopsis [array.syn]"),[<ranges>](ranges.syn#header:%3cranges%3e "25.2 Header <ranges> synopsis [ranges.syn]"), or[<utility>](utility.syn#header:%3cutility%3e "22.2.1 Header <utility> synopsis [utility.syn]") are included[.](#8.sentence-1)
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