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 type | Definition |
|---|---|
type | T1 if I == 0T2 if I == 1 |
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;
}; |
#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
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) |
| obtains the type of the specified element (class template specialization) |
|
obtains the type of the elements of array (class template specialization) |
|
|
(C++11) | obtains the size of a pair (class template specialization) |
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