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std::pair<T1,T2>::pair - cppreference.com

pair();
(1) (constexpr since C++11)
(conditionally explicit since C++11)		 
pair( const T1& x, const T2& y );
(2) (constexpr since C++14)
(conditionally explicit since C++11)		 
template< class U1, class U2 >
pair( U1&& x, U2&& y );
(3) (since C++11)
(constexpr since C++14)
(until C++23)
(conditionally explicit)		 
template< class U1 = T1, class U2 = T2 >
constexpr pair( U1&& x, U2&& y );
(since C++23)
(conditionally explicit)		 
template< class U1, class U2 >
constexpr pair( pair<U1, U2>& p );
(4) (since C++23)
(conditionally explicit) 
template< class U1, class U2 >
pair( const pair<U1, U2>& p );
(5) (constexpr since C++14)
(conditionally explicit since C++11)		 
template< class U1, class U2 >
pair( pair<U1, U2>&& p );
(6) (constexpr since C++14)
(conditionally explicit since C++11)		 
template< class U1, class U2 >
constexpr pair( const pair<U1, U2>&& p );
(7) (since C++23)
(conditionally explicit) 
template< pair-like P >
constexpr pair ( P&& u );
(8) (since C++23)
(conditionally explicit) 
template< class... Args1, class... Args2 >
pair( std::piecewise_construct_t,
      std::tuple<Args1...> first_args,
      std::tuple<Args2...> second_args );
(9) (since C++11)
(constexpr since C++20)		 
pair( const pair& p ) = default;
(10) 
pair( pair&& p ) = default;
(11) (since C++11)

Constructs a new pair.

1) Default constructor. Value-initializes both elements of the pair, first and second.

This constructor participates in overload resolution if and only if std::is_default_constructible_v<T1> and std::is_default_constructible_v<T2> are both true.

This constructor is explicit if and only if either T1 or T2 is not implicitly default-constructible.

(since C++11)

2) Initializes first with x and second with y.

This constructor participates in overload resolution if and only if std::is_copy_constructible_v<T1> and std::is_copy_constructible_v<T2> are both true.

This constructor is explicit if and only if std::is_convertible_v<const T1&, T1> is false or std::is_convertible_v<const T2&, T2> is false.

(since C++11)

3) Initializes first with std::forward<U1>(x) and second with std::forward<U2>(y).

This constructor participates in overload resolution if and only if std::is_constructible_v<T1, U1> and std::is_constructible_v<T2, U2> are both true.

This constructor is explicit if and only if std::is_convertible_v<U1, T1> is false or std::is_convertible_v<U2, T2> is false.

4) Initializes first with p.first and second with p.second.

This constructor participates in overload resolution if and only if std::is_constructible_v<T1, U1&> and std::is_constructible_v<T2, U2&> are both true.

This constructor is explicit if and only if std::is_convertible_v<U1&, T1> is false or std::is_convertible_v<U2&, T2> is false.

This constructor is defined as deleted if the initialization of first or second would bind a reference to temporary object.

5) Initializes first with p.first and second with p.second.

This constructor participates in overload resolution if and only if std::is_constructible_v<T1, const U1&> and std::is_constructible_v<T2, const U2&> are both true.

This constructor is explicit if and only if std::is_convertible_v<const U1&, T1> is false or std::is_convertible_v<const U2&, T2> is false.

(since C++11)

This constructor is defined as deleted if the initialization of first or second would bind a reference to temporary object.

(since C++23)

6) Initializes first with std::forward<U1>(p.first) and second with std::forward<U2>(p.second).

This constructor participates in overload resolution if and only if std::is_constructible_v<T1, U1> and std::is_constructible_v<T2, U2> are both true.

This constructor is explicit if and only if std::is_convertible_v<U1, T1> is false or std::is_convertible_v<U2, T2> is false.

This constructor is defined as deleted if the initialization of first or second would bind a reference to temporary object.

(since C++23)

7) Initializes first with std::forward<const U1>(p.first) and second with std::forward<const U2>(p.second).

This constructor participates in overload resolution if and only if std::is_constructible_v<T1, U1> and std::is_constructible_v<T2, U2> are both true.

This constructor is explicit if and only if std::is_convertible_v<const U1, T1> is false or std::is_convertible_v<const U2, T2> is false.

This constructor is defined as deleted if the initialization of first or second would bind a reference to temporary object.

8) Given u1 as std::get<0>(std::forward(u)) and u2 as std::get<1>(std::forward(u)), denote their types as U1 and U2 respectively. Initializes first with u1 and second with u2.

This constructor participates in overload resolution if and only if

  • std::remove_cvref(P) is not a specialization of std::ranges::subrange,
  • std::is_constructible_v<T1, U1> is true, and
  • std::is_constructible_v<T2, U2 is true.

This constructor is explicit if and only if std::is_convertible_v<U1, T1> is false or std::is_convertible_v<U2, T2> is false.

This constructor is defined as deleted if the initialization of first or second would bind a reference to temporary object.

9) Forwards the elements of first_args to the constructor of first and forwards the elements of second_args to the constructor of second. This is the only non-default constructor that can be used to create a pair of non-copyable non-movable types. The program is ill-formed if first or second is a reference and bound to a temporary object.

10) Copy constructor is implicitly declared(until C++11)defaulted, and is constexpr if copying of both elements satisfies the requirements on constexpr functions(since C++11).

11) Move constructor is defaulted, and is constexpr if moving of both elements satisfies the requirements on constexpr functions.

Parameters

x - value to initialize the first element of this pair
y - value to initialize the second element of this pair
p - pair of values used to initialize both elements of this pair
u - pair-like object of values used to initialize both elements of this pair
first_args - tuple of constructor arguments to initialize the first element of this pair
second_args - tuple of constructor arguments to initialize the second element of this pair

Exceptions

Does not throw exceptions unless one of the specified operations (e.g. constructor of an element) throws.

Example

#include <complex>
#include <iostream>
#include <string>
#include <tuple>
#include <utility>

int main()
{
    auto print = [](auto rem, auto const& pair)
    {
        std::cout << rem << "(" << pair.first << ", " << pair.second << ")\n";
    };
    
    std::pair<int, float> p1;
    print("(1) Value-initialized: ", p1);
    
    std::pair<int, double> p2{42, 3.1415};
    print("(2) Initialized with two values: ", p2);
    
    std::pair<char, int> p4{p2};
    print("(4) Implicitly converted: ", p4);
    
    std::pair<std::complex<double>, std::string> p6
        {std::piecewise_construct, std::forward_as_tuple(0.123, 7.7),
            std::forward_as_tuple(10, 'a')};
    print("(8) Piecewise constructed: ", p6);
}

Possible output: 

(1) Value-initialized: (0, 0)
(2) Initialized with two values: (42, 3.1415)
(4) Implicitly converted: (*, 3)
(8) Piecewise constructed: ((0.123,7.7), aaaaaaaaaa)

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 265 C++98 the default constructor copy-initialized first
and second with T1() and T2() respectively
(thus required T1 and T2 to be CopyConstructible) 		first and second
are value-initialized
LWG 2510 C++11 the default constructor was implicit made conditionally-explicit
N4387 C++11 some constructors were implicit-only, preventing some uses constructors made conditionally-explicit

See also

creates a pair object of type, determined by the argument types
(function template) [edit]
constructs a new tuple
(public member function of std::tuple<Types...>) [edit]