NAME¶

std::pair::pair - std::pair::pair

Synopsis¶

pair(); (until C++11)
(since C++11) constexpr pair(); (conditionally
explicit) pair( const T1& x, const (until C++11) T2& y );
(since C++11) pair( const T1& x, const (until C++14) T2& y ); (conditionally
explicit) constexpr pair( const T1& (since C++14) x, const T2& y ); (conditionally
explicit) template< class U1, class (since C++11) U2 > (until C++14) pair( U1&& x, U2&& y ); (conditionally
explicit) template< class U1, class (since C++14) U2 > (until C++23) constexpr pair( U1&& x, (conditionally U2&& y ); explicit) template< class U1 = T1, (since C++23) class U2 = T2 > (conditionally constexpr pair( U1&& x, explicit) U2&& y ); template< class U1, class (since C++23) U2 > (4) (conditionally constexpr pair( pair<U1, explicit) U2>& p ); template< class U1, class U2 > (until C++11) pair( const pair<U1, U2>& p ); template< class U1, class (since C++11) U2 > (until C++14) pair( const pair<U1, U2>& p (conditionally ); explicit) template< class U1, class (since C++14) U2 > (1) (conditionally constexpr pair( const explicit) pair<U1, U2>& p ); (2) template< class U1, class (since C++11) U2 > (until C++14) pair( pair<U1, U2>&& p ); (conditionally
(3) explicit) template< class U1, class (since C++14) U2 > (conditionally constexpr pair( pair<U1, explicit) U2>&& p ); template< class U1, class (since C++23) U2 > (7) (conditionally constexpr pair( const explicit) pair<U1, U2>&& p ); (5) template< class... Args1, class... Args2 >

pair( (since std::piecewise_construct_t, C++11) std::tuple<Args1...> (6) (until first_args, C++20)

std::tuple<Args2...> second_args ); template< class... Args1, class... Args2 > (8)

constexpr pair( std::piecewise_construct_t, (since std::tuple<Args1...> C++20) first_args,

std::tuple<Args2...> second_args ); pair( const pair& p ) = (9) default; pair( pair&& p ) = default; (10) (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. (since C++11)
* This constructor is explicit if and only if either T1 or T2 is not
implicitly default-constructible.

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. (since C++11)
* 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.

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 (since C++23)
first or second would bind a reference to temporary object.

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. (since C++11)
* 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 (since C++23)
first or second would bind a reference to temporary object.

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 (since C++23)
first or second would bind a reference to temporary object.

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) 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.
9) 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).
10) 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()
{
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
N4387 C++11 some constructors were constructors made
implicit-only, preventing some uses conditionally-explicit
LWG 2510 C++11 default constructor was implicit made conditionally-explicit

See also¶

make_pair creates a pair object of type, defined by the argument types
(function template)
constructor constructs a new tuple
(C++11) (public member function of std::tuple<Types...>)