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)

std::swap (C++11)

get (C++11)

Deduction guides(C++17)

Helper classes

tuple_size (C++11)

tuple_element (C++11)

basic_common_reference (C++23)

common_type (C++23)

 

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 <tuple>
#include <iostream>
#include <string>
 
namespace detail {
 
template <std::size_t>
struct index_tag { explicit constexpr 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>
auto constexpr get_val(std::pair<T, U> const& pair)
    -> typename std::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)