(1)  
pair();  (until C++11)  
constexpr pair();  (since C++11) (conditionally explicit)  
(2)  
pair( const T1& x, const T2& y );  (until C++11)  
pair( const T1& x, const T2& y );  (since C++11) (until C++14) (conditionally explicit)  
constexpr pair( const T1& x, const T2& y );  (since C++14) (conditionally explicit)  
(3)  
template< class U1, class U2 > pair( U1&& x, U2&& y );  (since C++11) (until C++14) (conditionally explicit)  
template< class U1, class U2 > constexpr pair( U1&& x, U2&& y );  (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) 
(5)  
template< class U1, class U2 > pair( const pair<U1, U2>& p );  (until C++11)  
template< class U1, class U2 > pair( const pair<U1, U2>& p );  (since C++11) (until C++14) (conditionally explicit)  
template< class U1, class U2 > constexpr pair( const pair<U1, U2>& p );  (since C++14) (conditionally explicit)  
(6)  
template< class U1, class U2 > pair( pair<U1, U2>&& p );  (since C++11) (until C++14) (conditionally explicit)  
template< class U1, class U2 > constexpr pair( pair<U1, U2>&& p );  (since C++14) (conditionally explicit)  
template< class U1, class U2 > constexpr pair( const pair<U1, U2>&& p );  (7)  (since C++23) (conditionally explicit) 
template< pairlike P > constexpr pair ( P&& u );  (8)  (since C++23) (conditionally explicit) 
(9)  
template< class... Args1, class... Args2 > pair( std::piecewise_construct_t, std::tuple<Args1...> first_args, std::tuple<Args2...> second_args );  (since C++11) (until C++20)  
template< class... Args1, class... Args2 > constexpr pair( std::piecewise_construct_t, std::tuple<Args1...> first_args, std::tuple<Args2...> second_args );  (since C++20)  
pair( const pair& p ) = default;  (10)  
pair( pair&& p ) = default;  (11)  (since C++11) 
Constructs a new pair.
first
and second
. This constructor participates in overload resolution if and only if This constructor is explicit if and only if either  (since C++11) 
first
with x
and second
with y
. This constructor participates in overload resolution if and only if This constructor is explicit if and only if  (since C++11) 
std::is_constructible_v<T1, U1>
and std::is_constructible_v<T2, U2>
are both true
.explicit
first
with p.first
and second
with p.second
.std::is_constructible_v<T1, U1&>
and std::is_constructible_v<T2, U2&>
are both true
.explicit
first
or second
would bind a reference to temporary object.first
with p.first
and second
with p.second
. This constructor participates in overload resolution if and only if This constructor is explicit if and only if  (since C++11) 
This constructor is defined as deleted if the initialization of  (since C++23) 
std::is_constructible_v<T1, U1>
and std::is_constructible_v<T2, U2>
are both true
.explicit
first
with std::forward<const U1>(p.first)
and second
with std::forward<const U2>(p.second)
.std::is_constructible_v<T1, U1>
and std::is_constructible_v<T2, U2>
are both true
.explicit
first
or second
would bind a reference to temporary object.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
.std::ranges::subrange
, std::is_constructible_v<T1, U1>
is true
, and std::is_constructible_v<T2, U2
is true
. explicit
first
or second
would bind a reference to temporary object.first_args
to the constructor of first
and forwards the elements of second_args
to the constructor of second
. This is the only nondefault constructor that can be used to create a pair of noncopyable nonmovable types. The program is illformed if first
or second
is a reference and bound to a temporary object.constexpr
if copying of both elements satisfies the requirements on constexpr functions (since C++11).constexpr
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    pairlike 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 
Does not throw exceptions unless one of the specified operations (e.g. constructor of an element) throws.
#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) Valueinitialized: ", 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) Valueinitialized: (0, 0) (2) Initialized with two values: (42, 3.1415) (4) Implicitly converted: (*, 3) (8) Piecewise constructed: ((0.123,7.7), aaaaaaaaaa)
The following behaviorchanging 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 copyinitialized first and second with T1() and T2() respectively(thus required T1 and T2 to be CopyConstructible)  first and second are valueinitialized 
LWG 2510  C++11  the default constructor was implicit  made conditionallyexplicit 
N4387  C++11  some constructors were implicitonly, preventing some uses  constructors made conditionallyexplicit 
creates a pair object of type, defined by the argument types (function template) 

(C++11)  constructs a new tuple (public member function of std::tuple<Types...> ) 
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