Namespaces
Variants
Views
Actions

std::tuple_element<std::pair>

From cppreference.com
< cpp‎ | utility‎ | pair
 
 
Utilities library
General utilities
Date and time
Function objects
Formatting library (C++20)
(C++11)
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)
Swap and type operations
(C++14)
(C++11)
(C++11)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++17)

Elementary string conversions
(C++17)
(C++17)
 
std::pair
Member functions
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
(C++11)
Deduction guides(C++17)
Helper classes
(C++11)
tuple_element
(C++11)
 
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

Contents

[edit] Member types

Member type Definition
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 <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

[edit] 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)

[edit] See also

obtains the type of the specified element
(class template specialization) [edit]
obtains the type of the elements of array
(class template specialization) [edit]
obtains the size of a pair
(class template specialization) [edit]