std::pair::pair

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

make_from_tuple (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Elementary string conversions

to_chars (C++17)

from_chars (C++17)

chars_format (C++17)

Type operations

declval (C++11)

as_const (C++17)

 

std::pair

Member functions

pair::pair

pair::operator=

pair::swap

Non-member functions

make_pair

operator==operator!=operator<operator<=operator>operator>=

std::swap

get (C++11)

Helper classes

tuple_size (C++11)

tuple_element (C++11)

 

	(1)
pair();			(until C++11)
constexpr pair();			(since C++11) (until C++17)
/*EXPLICIT*/ constexpr pair();			(since C++17)
		(2)
pair( const T1& x, const T2& y );				(until C++14)
constexpr pair( const T1& x, const T2& y );				(since C++14) (until C++17)
/*EXPLICIT*/ constexpr pair( const T1& x, const T2& y );				(since C++17)
			(3)
template< class U1, class U2 > pair( U1&& x, U2&& y );					(since C++11) (until C++14)
template< class U1, class U2 > constexpr pair( U1&& x, U2&& y );					(since C++14) (until C++17)
template< class U1, class U2 > /*EXPLICIT*/ constexpr pair( U1&& x, U2&& y );					(since C++17)
				(4)
template< class U1, class U2 > pair( const pair<U1, U2>& p );						(until C++14)
template< class U1, class U2 > constexpr pair( const pair<U1, U2>& p );						(since C++14) (until C++17)
template< class U1, class U2 > /*EXPLICIT*/ constexpr pair( const pair<U1, U2>& p );						(since C++17)
					(5)
template< class U1, class U2 > pair( pair<U1, U2>&& p );							(since C++11) (until C++14)
template< class U1, class U2 > constexpr pair( pair<U1, U2>&& p );							(since C++14) (until C++17)
template< class U1, class U2 > /*EXPLICIT*/ constexpr pair( pair<U1, U2>&& p );							(since C++17)
template< class... Args1, class... Args2 > pair( std::piecewise_construct_t,       std::tuple<Args1...> first_args,       std::tuple<Args2...> second_args );						(6)	(since C++11)
pair( const pair& p ) = default;						(7)
pair( pair&& p ) = default;						(8)	(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<first_type> and std::is_default_constructible_v<second_type> are both true. This constructor is explicit if and only if either first_type or second_type is not implicitly default-constructible.

(since C++17)

2) Initializes first with x and second with y.

This constructor participates in overload resolution if and only if std::is_copy_constructible_v<first_type> and std::is_copy_constructible_v<second_type> are both true. This constructor is explicit if and only if std::is_convertible_v<const first_type&, first_type> is false or std::is_convertible_v<const second_type&, second_type> is false.

(since C++17)

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

This constructor participates in overload resolution only if U1 is implicitly convertible to first_type and U2 is implicitly convertible to second_type.	(until C++17)
This constructor participates in overload resolution if and only if std::is_constructible_v<first_type, U1&&> and std::is_constructible_v<second_type, U2&&> are both true. This constructor is explicit if and only if std::is_convertible_v<U1&&, first_type> is false or std::is_convertible_v<U2&&, second_type> is false.	(since C++17)

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<first_type, const U1&> and std::is_constructible_v<second_type, const U2&> are both true. This constructor is explicit if and only if std::is_convertible_v<const U1&, first_type> is false or std::is_convertible_v<const U2&, second_type> is false.

(since C++17)

5) 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<first_type, U1&&> and std::is_constructible_v<second_type, U2&&> are both true. This constructor is explicit if and only if std::is_convertible_v<U1&&, first_type> is false or std::is_convertible_v<U2&&, second_type> is false.

(since C++17)

6) 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.

7) Copy constructor is defaulted, and is constexpr if copying of both elements satisfies the requirements on constexpr functions.

8) 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
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

Run this code

#include <utility>
#include <string>
#include <complex>
#include <tuple>
#include <iostream>
 
int main()
{
    std::pair<int, float> p1;
    std::cout << "Value-initialized: "
              << p1.first << ", " << p1.second << '\n';
 
    std::pair<int, double> p2(42, 0.123);
    std::cout << "Initialized with two values: "
              << p2.first << ", " << p2.second << '\n';
 
    std::pair<char, int> p4(p2);
    std::cout << "Implicitly converted: "
              << p4.first << ", " << p4.second << '\n';
 
    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'));
    std::cout << "Piecewise constructed: "
              << p6.first << ", " << p6.second << '\n';
}

Output:

Value-initialized: 0, 0
Initialized with two values: 42, 0.123
Implicitly converted: *, 0
Piecewise constructed: (0.123,7.7), aaaaaaaaaa

See also

make_pair

creates a pair object of type, defined by the argument types (function template)