23 General utilities library [utilities]

23.5 Tuples [tuple]

23.5.3 Class template tuple [tuple.tuple]

23.5.3.7 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;

1
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Requires: I < sizeof...(Types). The program is ill-formed if I is out of bounds.

2
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Returns: A reference to the Ith element of t, where indexing is zero-based.

3
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[ Note A: If a 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 ]

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[ Note B: Constness is shallow. If a 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 member variables 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;

5
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Requires: The type T occurs exactly once in Types.... Otherwise, the program is ill-formed.

6
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Returns: A reference to the element of t corresponding to the type T in Types....

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[ Example: 

  const tuple<int, const int, double, double> t(1, 2, 3.4, 5.6);
  const int& i1 = get<int>(t);        // OK. Not ambiguous. i1 == 1
  const int& i2 = get<const int>(t);  // OK. Not ambiguous. i2 == 2
  const double& d = get<double>(t);   // ERROR. ill-formed

 — end example ]

8
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[ Note: 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 ]