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.
type T1 if I == 0
T2 if I == 1 [edit] 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;
};[edit] Example
#include <iostream>
#include <string>
#include <tuple>
namespace detail
{
template<std::size_t>
struct index_tag { constexpr explicit 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:
[edit] Defect reportsThe 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) [edit] See alsoRetroSearch is an open source project built by @garambo | Open a GitHub Issue
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