NAME

std::tuple_element - std::tuple_element

Synopsis

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

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 made ill-formed (hard
primary template 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> obtains the type of the specified element
(C++11) (class template specialization)
std::tuple_element<std::array> obtains the type of the elements of array
(C++11) (class template specialization)
std::tuple_element<std::ranges::subrange> obtains the type of the iterator or the
(C++20) sentinel of a std::ranges::subrange
(class template specialization)
std::tuple_size<std::pair> obtains the size of a pair
(C++11) (class template specialization)