std::pair::pair

From Cppreference


pair(); constexpr pair();	(1)	(pre-C++11 version) (C++11 version)

pair( const T1& x, const T2& y );	(2)

template< class U1, class U2 > pair( U1&& x, U1&& y );	(3)	(C++11 feature)

template< class U1, class U2 > pair( const pair<U1,U2>& p );	(4)

template< class U1, class U2 > pair( pair<U1,U2>&& p );	(5)	(C++11 feature)

template< class... Args1, class... Args2 > pair( std::piecewise_construct_t, std::tuple<Args1...> first_args, std::tuple<Args2...> second_args );	(6)	(C++11 feature)

pair( const pair& p ) = default;	(7)

pair( pair&& p ) = default;	(8)	(C++11 feature)

Constructs a new pair.

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

2) Initializes first with x and second with y.

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

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

5) Initializees first with std::forward<U>(p.first) and second with std::forward<V>(p.second).

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 implicitly generated.

8) Move constructor is implicitly generated.

[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
`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] Example

#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

[edit] See also

make_pair

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