23 General utilities library [utilities]

23.5 Tuples [tuple]

23.5.3 Class template tuple [tuple.tuple]

23.5.3.6 Tuple helper classes [tuple.helper]

template<class T> struct tuple_size;
Remarks: All specializations of tuple_­size shall meet the UnaryTypeTrait requirements with a base characteristic of integral_­constant<size_­t, N> for some N.
template<class... Types> class tuple_size<tuple<Types...>> : public integral_constant<size_t, sizeof...(Types)> { }; 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 Ith element of Types, where indexing is zero-based.
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 meet the UnaryTypeTrait requirements 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.
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 meet the TransformationTrait requirements with a member typedef type that names the following type:
  • for the first specialization, add_­const_­t<TE>,
  • for the second specialization, add_­volatile_­t<TE>, and
  • 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.