std::pair<T1,T2>::pair
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, |
(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.
first and second.
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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 |
(since C++11) |
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) |
|
This constructor is defined as deleted if the initialization of |
(since C++23) |
first with p.first and second with p.second.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 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 |
(since C++23) |
|
This constructor is defined as deleted if the initialization of |
(since C++23) |
first with std::forward<const U1>(p.first) and second with std::forward<const U2>(p.second).first or second would bind a reference to temporary object.U1 and U2 respectively. Initializes first with u1 and second with u2.- std::remove_cvref(P) is not a specialization of std::ranges::subrange,
- std::is_constructible_v<T1, U1> is true, and
- std::is_constructible_v<T2, U2 is true.
first or second would bind a reference to temporary object.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.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 |
Exceptions
Does not throw exceptions unless one of the specified operations (e.g. constructor of an element) throws.
Example
#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)
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 firstand second with T1() and T2() respectively(thus required T1 and T2 to be CopyConstructible)
|
first and secondare 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 |
See also
creates a pair object of type, determined by the argument types (function template) | |
constructs a new tuple (public member function of std::tuple<Types...>) |