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std::pair::pair(3) C++ Standard Libary std::pair::pair(3) 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 > (un- til C++14) pair( U1&& x, U2&& y ); (conditionally ex- plicit) 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 > (conditionally constexpr pair( const explicit) pair<U1, U2>& p ); (1) template< class U1, class (since C++11) U2 > (2) (until C++14) pair( pair<U1, U2>&& p ); (conditionally ex- plicit) template< class U1, class (since C++14) U2 > (conditionally constexpr pair( pair<U1, (3) explicit) U2>&& p ); template< class U1, class (since C++23) U2 > (7) (conditionally constexpr pair( const explicit) pair<U1, U2>&& p ); template< pair-like P > (since C++23) constexpr pair ( P&& u ); (8) (conditionally (5) explicit) 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 > (9) constexpr pair( std::piecewise_construct_t, (since std::tuple<Args1...> C++20) first_args, std::tuple<Args2...> second_args ); pair( const pair& p ) = (10) default; pair( pair&& p ) = default; (11) (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_con- structible_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_convert- ible_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 (since C++23) 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_convert- ible_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_con- structible_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 first (since C++23) 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_convert- ible_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 (since C++23) 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_convert- ible_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) Given 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. This constructor participates in overload resolution if and only if * 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. This constructor is explicit if and only if std::is_convert- ible_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. 9) 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. 10) 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). 11) 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 u - pair-like object of values used to initialize both el- ements 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 sec- ond 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 <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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior the default constructor copy-initialized first LWG 265 C++98 and second with T1() and T2() first and second respectively are value- initialized (thus required T1 and T2 to be CopyConstructible) LWG 2510 C++11 the default constructor was made condi- tionally-explicit implicit N4387 C++11 some constructors were constructors made implicit-only, preventing some uses condition- ally-explicit See also make_pair creates a pair object of type, defined by the argument types (function template) constructor constructs a new tuple (public member function of std::tuple<Types...>) http://cppreference.com 2024.06.10 std::pair::pair(3)
NAME | Synopsis | Parameters | Exceptions | Example | Possible output: | See also
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