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std::pair<T1,T2>::pair

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< cpp‎ | utility‎ | pair
 
 
Utilities library
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Three-way comparison
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(C++20)(C++20)(C++20)  
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General utilities
Relational operators (deprecated in C++20)
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(C++20)(C++20)(C++20)  
(C++20)
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(C++11)
(C++17)
(C++23)


 
 
pair();
(1) (constexpr since C++11)
(conditionally explicit since C++11)
pair( const T1& x, const T2& y );
(2) (conditionally explicit since C++11)
(constexpr since C++14)
(3)
template< class U1, class U2 >
pair( U1&& x, U2&& y );
(since C++11)
(until C++23)
(constexpr since C++14)
(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) (conditionally explicit since C++11)
(constexpr since C++14)
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.

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

(since C++23)
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
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.

Contents

[edit] 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

[edit] Exceptions

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

[edit] Example

Run this code
#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)

[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 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

[edit] 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]

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