20 General utilities library [utilities]

20.5 Tuples [tuple]

20.5.6 Tuple helper classes [tuple.helper]

template<class T> struct tuple_size;

All specializations of tuple_size meet the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of integral_constant<size_t, N> for some N.

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template<class... Types>
  struct tuple_size<tuple<Types...>> : public integral_constant<size_t, sizeof...(Types)> { };

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template<size_t I, class... Types>
  struct tuple_element<I, tuple<Types...>> {
    using type = TI;
  };

Mandates:

I < sizeof...(Types)

Type: TI is the type of the

I^{th}

element of Types, where indexing is zero-based.

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template<class T> struct tuple_size<const T>;

Let TS denote tuple_size<T> of the cv-unqualified type T.

If the expression TS::value is well-formed when treated as an unevaluated operand, then each specialization of the template meets the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of

integral_constant<size_t, TS::value>

Otherwise, it has no member value.

Access checking is performed as if in a context unrelated to TS and T.

Only the validity of the immediate context of the expression is considered.

[ Note

The compilation of the expression can result in side effects such as the instantiation of class template specializations and function template specializations, the generation of implicitly-defined functions, and so on.

Such side effects are not in the “immediate context” and can result in the program being ill-formed.

— end note

]

In addition to being available via inclusion of the <tuple> header, the template is available when any of the headers <array> ([array.syn]), <ranges> ([ranges.syn]), or <utility> ([utility.syn]) are included.

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template<size_t I, class T> struct tuple_element<I, const T>;

Let TE denote tuple_element_t<I, T> of the cv-unqualified type T.

Then each specialization of the template meets the Cpp17TransformationTrait requirements ([meta.rqmts]) with a member typedef type that names the type add_const_t<TE>.