std::tuple_element<std::pair>

 

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std::pair

Member functions

pair::pair

pair::operator=

pair::swap (C++11)

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::pair) (C++11)

get(std::pair) (C++11)

Helper classes

tuple_size<std::pair> (C++11)

tuple_element<std::pair> (C++11)

basic_common_reference<std::pair> (C++23)

common_type<std::pair> (C++23)

formatter<std::pair> (C++23)

piecewise_construct_t (C++11)

Deduction guides(C++17)

 

Defined in header <utility>
template<std::size_t I, class T1, class T2 > struct tuple_element<I, std::pair<T1, T2>>;		(since C++11)

The partial specializations of std::tuple_element for pairs provide compile-time access to the types of the pair's elements, using tuple-like syntax. The program is ill-formed if I >= 2.

Member types

Member type	Definition
type	T1 if I == 0 T2 if I == 1

Possible implementation

template<std::size_t I, typename T>struct tuple_element;   template<std::size_t I, typename T1, typename T2>struct tuple_element<I, std::pair<T1, T2>>{ static_assert(I <2, "std::pair has only 2 elements!");};   template<typename T1, typename T2>struct tuple_element<0, std::pair<T1, T2>>{using type = T1;};   template<typename T1, typename T2>struct tuple_element<1, std::pair<T1, T2>>{using type = T2;};

Example

Run this code

#include <iostream>#include <string>#include <tuple>   namespace detail {template<std::size_t>struct index_tag {constexprexplicit index_tag()=default;};   template<class T, class U>constexpr T get_val_dispatch(std::pair<T, U>const& pair, index_tag<0>){return pair.first;}   template<class T, class U>constexpr U get_val_dispatch(std::pair<T, U>const& pair, index_tag<1>){return pair.second;}}// namespace detail   template<std::size_t N, class T, class U>autoconstexpr get_val(std::pair<T, U>const& pair)->typenamestd::tuple_element<N, std::pair<T, U>>::type{return detail::get_val_dispatch(pair, detail::index_tag<N>{});}   int main(){auto var =std::make_pair(1, std::string{"one"});   std::cout<< get_val<0>(var)<<" = "<< get_val<1>(var);}

Output:

1 = one

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2974	C++11	out-of-bounds index referred the undefined primary template	made ill-formed (hard error)

See also

Structured binding(C++17)	binds the specified names to sub-objects or tuple elements of the initializer
std::tuple_element<std::tuple> (C++11)	obtains the type of the specified element (class template specialization)
std::tuple_element<std::array> (C++11)	obtains the type of the elements of array (class template specialization)
std::tuple_element<std::ranges::subrange> (C++20)	obtains the type of the iterator or the sentinel of a std::ranges::subrange (class template specialization)
std::tuple_size<std::pair> (C++11)	obtains the size of a pair (class template specialization)