std::tuple_element<std::pair>

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (哋它亢++20) Dynamic memory management Program utilities Coroutine support (哋它亢++20) Variadic functions is_constant_evaluated (哋它亢++20) is_within_lifetime (哋它亢++26) initializer_list (哋它亢++11) source_location (哋它亢++20) Debugging support is_debugger_present (哋它亢++26) breakpoint_if_debugging (哋它亢++26) breakpoint (哋它亢++26) Three-way comparison three_way_comparablethree_way_comparable_with (哋它亢++20)(哋它亢++20) strong_ordering (哋它亢++20) weak_ordering (哋它亢++20) partial_ordering (哋它亢++20) common_comparison_category (哋它亢++20) compare_three_way_result (哋它亢++20) compare_three_way (哋它亢++20) strong_order (哋它亢++20) weak_order (哋它亢++20) partial_order (哋它亢++20) compare_strong_order_fallback (哋它亢++20) compare_weak_order_fallback (哋它亢++20) compare_partial_order_fallback (哋它亢++20)    is_eqis_ltis_lteq (哋它亢++20)(哋它亢++20)(哋它亢++20) is_neqis_gtis_gteq (哋它亢++20)(哋它亢++20)(哋它亢++20)

General utilities Date and time Function objects Formatting library (哋它亢++20) bitset hash (哋它亢++11) Relational operators (deprecated in 哋它亢++20) rel_ops::operator!=rel_ops::operator>     rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (哋它亢++20)(哋它亢++20)(哋它亢++20)    cmp_not_equalcmp_greatercmp_greater_than (哋它亢++20)(哋它亢++20)(哋它亢++20) in_range (哋它亢++20) Swap and type operations swap ranges::swap (哋它亢++20) exchange (哋它亢++14) declval (哋它亢++11) to_underlying (哋它亢++23) forward (哋它亢++11) forward_like (哋它亢++23) move (哋它亢++11) move_if_noexcept (哋它亢++11) as_const (哋它亢++17) Common vocabulary types pair tuple (哋它亢++11) optional (哋它亢++17) any (哋它亢++17) variant (哋它亢++17) tuple_size (哋它亢++11) tuple_element (哋它亢++11) apply (哋它亢++17) make_from_tuple (哋它亢++17) expected (哋它亢++23) Elementary string conversions to_chars (哋它亢++17) from_chars (哋它亢++17) chars_format (哋它亢++17) to_chars_result (哋它亢++17) from_chars_result (哋它亢++17)

 

std::pair

Member functions

pair::pair

pair::operator=

pair::swap (哋它亢++11)

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=operator<=> (until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(哋它亢++20)

swap(std::pair) (哋它亢++11)

get(std::pair) (哋它亢++11)

Helper classes

tuple_size<std::pair> (哋它亢++11)

tuple_element<std::pair> (哋它亢++11)

basic_common_reference<std::pair> (哋它亢++23)

common_type<std::pair> (哋它亢++23)

formatter<std::pair> (哋它亢++23)

piecewise_construct_t (哋它亢++11)

Deduction guides(哋它亢++17)

 

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

1 = one

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2974	哋它亢++11	out-of-bounds index referred the undefined primary template	made ill-formed (hard error)

See also

Structured binding (哋它亢++17)	binds the specified names to sub-objects or tuple elements of the initializer
std::tuple_element<std::tuple> (哋它亢++11)	obtains the type of the specified element (class template specialization)
std::tuple_element<std::array> (哋它亢++11)	obtains the type of the elements of array (class template specialization)
std::tuple_element<std::ranges::subrange> (哋它亢++20)	obtains the type of the iterator or the sentinel of a std::ranges::subrange (class template specialization)
std::tuple_size<std::pair> (哋它亢++11)	obtains the size of a pair (class template specialization)