22 General utilities library [utilities]

22.4 Tuples [tuple]

22.4.8 Element access [tuple.elem]

template<size_t I, class... Types>
  constexpr tuple_element_t<I, tuple<Types...>>&
    get(tuple<Types...>& t) noexcept;
template<size_t I, class... Types>
  constexpr tuple_element_t<I, tuple<Types...>>&&
    get(tuple<Types...>&& t) noexcept;        // Note A
template<size_t I, class... Types>
  constexpr const tuple_element_t<I, tuple<Types...>>&
    get(const tuple<Types...>& t) noexcept;   // Note B
template<size_t I, class... Types>
  constexpr const tuple_element_t<I, tuple<Types...>>&& get(const tuple<Types...>&& t) noexcept;

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

Returns: A reference to the

I^{th}

element of t, where indexing is zero-based.

[Note A 1:

If a type T in Types is some reference type X&, the return type is X&, not X&&.

However, if the element type is a non-reference type T, the return type is T&&.

— end note]

[Note B 2:

Constness is shallow.

If a type T in Types is some reference type X&, the return type is X&, not const X&.

However, if the element type is a non-reference type T, the return type is const T&.

This is consistent with how constness is defined to work for non-static data members of reference type.

— end note]

🔗

template<class T, class... Types>
  constexpr T& get(tuple<Types...>& t) noexcept;
template<class T, class... Types>
  constexpr T&& get(tuple<Types...>&& t) noexcept;
template<class T, class... Types>
  constexpr const T& get(const tuple<Types...>& t) noexcept;
template<class T, class... Types>
  constexpr const T&& get(const tuple<Types...>&& t) noexcept;

Mandates: The type T occurs exactly once in Types.

Returns: A reference to the element of t corresponding to the type T in Types.

[Example 1: const tuple<int, const int, double, double> t(1, 2, 3.4, 5.6); const int& i1 = get<int>(t); // OK, i1 has value 1 const int& i2 = get<const int>(t); // OK, i2 has value 2 const double& d = get<double>(t); // error: type double is not unique within t — end example]

[Note 3:

The reason get is a non-member function is that if this functionality had been provided as a member function, code where the type depended on a template parameter would have required using the template keyword.

— end note]