19 General utilities library [utilities]

19.5 Tuples [tuple]

19.5.3 Class template tuple [tuple.tuple]

19.5.3.6 Tuple helper classes [tuple.helper]

template<class T> struct tuple_size;

Remarks: All specializations of tuple_size shall satisfy 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>
  class tuple_size<tuple<Types...>> : public integral_constant<size_t, sizeof...(Types)> { };

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

Requires: I < sizeof...(Types).

The program is ill-formed if I is out of bounds.

Type: TI is the type of the I

^{th}

element of Types, where indexing is zero-based.

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template<class T> class tuple_size<const T>;
template<class T> class tuple_size<volatile T>;
template<class T> class tuple_size<const volatile 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 of the three templates shall satisfy the Cpp17UnaryTypeTrait requirements ([meta.rqmts]) with a base characteristic of

integral_constant<size_t, TS::value>

Otherwise, they shall have 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 three templates are available when either of the headers <array> or <utility> are included.

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

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

Then each of the three templates shall satisfy the Cpp17TransformationTrait requirements ([meta.rqmts]) with a member typedef type that names the following type:

(7.1)
for the first specialization, add_const_t<TE>,
(7.2)
for the second specialization, add_volatile_t<TE>, and
(7.3)
for the third specialization, add_cv_t<TE>.

In addition to being available via inclusion of the <tuple> header, the three templates are available when either of the headers <array> or <utility> are included.