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std::tuple::tuple(3) C++ Standard Libary std::tuple::tuple(3) NAME std::tuple::tuple - std::tuple::tuple Synopsis Defined in header <tuple> constexpr tuple(); (1) (since C++11) (condi- tionally explicit) (since C++11) tuple( const Types&... args ); (2) (const- expr since C++14) (condi- tionally explicit) template< class... UTypes > (since C++11) tuple( UTypes&&... args ); (3) (const- expr since C++14) (condi- tionally explicit) template< class... UTypes > (4) (since C++23) constexpr tuple( tuple<UTypes...>& other ); (condi- tionally explicit) template< class... UTypes > (since C++11) tuple( const tuple<UTypes...>& other ); (5) (const- expr since C++14) (condi- tionally explicit) template< class... UTypes > (since C++11) tuple( tuple<UTypes...>&& other ); (6) (const- expr since C++14) (condi- tionally explicit) template< class... UTypes > (7) (since C++23) constexpr tuple( const tuple<UTypes...>&& other ); (condi- tionally explicit) template< class U1, class U2 > (8) (since C++23) constexpr tuple( std::pair<U1, U2>& p ); (condi- tionally explicit) template< class U1, class U2 > (since C++11) tuple( const std::pair<U1, U2>& p ); (9) (const- expr since C++14) (condi- tionally explicit) template< class U1, class U2 > (since C++11) tuple( std::pair<U1, U2>&& p ); (10) (const- expr since C++14) (condi- tionally explicit) template< class U1, class U2 > (11) (since C++23) constexpr tuple( const std::pair<U1, U2>&& p ); (condi- tionally explicit) template< tuple-like UTuple > (12) (since C++23) constexpr tuple( UTuple&& u ); (condi- tionally explicit) tuple( const tuple& other ) = default; (13) (since C++11) tuple( tuple&& other ) = default; (14) (since C++11) Allocator-extended constructors template< class Alloc > (since C++11) tuple( std::allocator_arg_t, const Alloc& a ); (15) (const- expr since C++20) (condi- tionally explicit) template< class Alloc > (since C++11) tuple( std::allocator_arg_t, const Alloc& a, (16) (const- expr since C++20) (condi- tionally explicit) const Types&... args ); template< class Alloc, class... UTypes > (since C++11) tuple( std::allocator_arg_t, const Alloc& a, (17) (const- expr since C++20) (condi- tionally explicit) UTypes&&... args ); template< class Alloc, class... UTypes > (since C++23) constexpr tuple( std::allocator_arg_t, const Alloc& a, (18) (condi- tionally explicit) tuple<UTypes...>& other ); template< class Alloc, class... UTypes > (since C++11) tuple( std::allocator_arg_t, const Alloc& a, (19) (const- expr since C++20) (condi- tionally explicit) const tuple<UTypes...>& other ); template< class Alloc, class... UTypes > (since C++11) tuple( std::allocator_arg_t, const Alloc& a, (20) (const- expr since C++20) (condi- tionally explicit) tuple<UTypes...>&& other ); template< class Alloc, class... UTypes > (since C++23) constexpr tuple( std::allocator_arg_t, const Alloc& a, (21) (condi- tionally explicit) const tuple<UTypes...>&& other ); template< class Alloc, class U1, class U2 > (since C++23) constexpr tuple( std::allocator_arg_t, const Alloc& a, (22) (condi- tionally explicit) std::pair<U1, U2>& p ); template< class Alloc, class U1, class U2 > (since C++11) tuple( std::allocator_arg_t, const Alloc& a, (23) (const- expr since C++20) (condi- tionally explicit) const std::pair<U1, U2>& p ); template< class Alloc, class U1, class U2 > (since C++11) tuple( std::allocator_arg_t, const Alloc& a, (24) (const- expr since C++20) (condi- tionally explicit) std::pair<U1, U2>&& p ); template< class Alloc, class U1, class U2 > (since C++23) constexpr tuple( std::allocator_arg_t, const Alloc& a, (25) (condi- tionally explicit) const std::pair<U1, U2>&& p ); template< class Alloc, tuple-like UTuple > (since C++23) constexpr tuple( std::allocator_arg_t, const Alloc& a, (26) (condi- tionally explicit) UTuple&& u ); template< class Alloc > (since C++11) tuple( std::allocator_arg_t, const Alloc& a, (27) (const- expr since C++20) const tuple& other ); template< class Alloc > (since C++11) tuple( std::allocator_arg_t, const Alloc& a, (28) (const- expr since C++20) tuple&& other ); Constructs a new tuple. In the descriptions that follow, let * i be in the range [0, sizeof...(Types)) in order, * Ti be the ith type in Types, and * Ui be the ith type in a template parameter pack named UTypes, where indexing is zero-based. 1) Default constructor. Value-initializes all elements, if any. The default constructor is trivial if sizeof...(Types) == 0. * This overload participates in overload resolution only if std::is_default_constructible<Ti>::value is true for all i. * The constructor is explicit if and only if Ti is not copy-list- initializable from {} for at least one i. 2) Direct constructor. Initializes each element of the tuple with the corresponding parameter. * This overload participates in overload resolution only if sizeof...(Types) >= 1 and std::is_copy_constructible<Ti>::value is true for all i. * This constructor is explicit if and only if std::is_convert- ible<const Ti&, Ti>::value is false for at least one i. 3) Converting constructor. Initializes each element of the tuple with the corresponding value in std::forward<UTypes>(args). * This overload participates in overload resolution only if * sizeof...(Types) == sizeof...(UTypes), * sizeof...(Types) >= 1, * std::is_constructible<Ti, Ui>::value is true for all i, and * let D be std::decay<U0>::type (until C++20) std::remove_cvref_t<U0> (since C++20), * if sizeof...(Types) == 1, then D is not std::tuple, otherwise, * if sizeof...(Types) == 2 or sizeof...(Types) == 3, then either D is not std::allocator_arg_t, or T0 is std::alloca- tor_arg_t. * The constructor is explicit if and only if std::is_convert- ible<Ui, Ti>::value is false for at least one i. * This constructor is defined as deleted if the initialization of any element that is a reference would bind it to a temporary (since C++23) object. 4-7) Converting constructor. Initializes each element of the tuple with the corresponding element of other. Formally, let FWD(other) be std::forward<decltype(other)>(other), for all i, initializes ith element of the tuple with std::get<i>(FWD(other)). * This overload participates in overload resolution only if * sizeof...(Types) == sizeof...(UTypes), * std::is_constructible_v<Ti, de- cltype(std::get<i>(FWD(other)))> is true for all i, and * either * sizeof...(Types) is not 1, or * (when Types... expands to T and UTypes... expands to U) std::is_convertible_v<decltype(other), T>, std::is_constructible_v<T, decltype(other)>, and std::is_same_v<T, U> are all false. * These constructors are explicit if and only if std::is_convertible_v<decltype(std::get<i>(FWD(other))), Ti> is false for at least one i. * These constructors are defined as deleted if the initialization of any element that is a reference would bind it to a temporary (since C++23) object. 8-11) Pair constructor. Constructs a 2-element tuple with each ele- ment constructed from the corresponding element of p. Formally, let FWD(p) be std::forward<decltype(p)>(p), initializes the first element with std::get<0>(FWD(p)) and the second element with std::get<1>(FWD(p)). * This overload participates in overload resolution only if * sizeof...(Types) == 2, * std::is_constructible_v<T0, decltype(std::get<0>(FWD(p)))> is true, and * std::is_constructible_v<T1, decltype(std::get<1>(FWD(p)))> is true. * The constructor is explicit if and only if std::is_convertible_v<decltype(std::get<0>(FWD(p))), T0> or std::is_convertible_v<decltype(std::get<1>(FWD(p))), T1> is false. * These constructors are defined as deleted if the initialization of any element that is a reference would bind it to a temporary (since C++23) object. 12) tuple-like constructor. Constructs a tuple with each element constructed from the corresponding element of u. Formally, for all i, initializes ith element of the tuple with std::get<i>(std::forward<UTuple>(u)). * This overload participates in overload resolution only if * std::same_as<std::remove_cvref_t<UTuple>, std::tuple> is false, * std::remove_cvref_t<UTuple> is not a specialization of std::ranges::subrange, * sizeof...(Types) equals std::tuple_size_v<std::re- move_cvref_t<UTuple>>, * std::is_constructible_v<Ti, decltype(std::get<i>(std::for- ward<UTuple>(u)))> is true for all i, and * either * sizeof...(Types) is not 1, or * (when Types... expands to T) std::is_convert- ible_v<UTuple, T> and std::is_constructible_v<T, UTuple> are both false. * This constructor is defined as deleted if the initialization of any element that is a reference would bind it to a temporary object. 13) Implicitly-defined copy constructor. Initializes each element of the tuple with the corresponding element of other. * This constructor is constexpr if every operation it performs is constexpr. For the empty tuple std::tuple<>, it is constexpr. * std::is_copy_constructible<Ti>::value must be true for all i, otherwise the behavior is undefined (until C++20) the program is ill-formed (since C++20). 14) Implicitly-defined move constructor. For all i, initializes the ith element of the tuple with std::forward<Ui>(std::get<i>(other)). * This constructor is constexpr if every operation it performs is constexpr. For the empty tuple std::tuple<>, it is constexpr. * std::is_move_constructible<Ti>::value must be true for all i, otherwise the behavior is undefined (until C++20) this overload does not participate in overload resolution (since C++20). 15-28) Identical to (1-14) except each element is created by uses- allocator construction, that is, the Allocator object a is passed as an addi- tional argument to the constructor of each element for which std::uses_allocator<Ui, Alloc>::value is true. Parameters args - values used to initialize each element of the tuple other - the tuple of values used to initialize each element of the tuple p - the pair of values used to initialize both elements of the 2-tuple u - the tuple-like object of values used to initialize each ele- ment of the tuple a - the allocator to use in uses-allocator construction Notes Conditionally-explicit constructors make it possible to construct a tuple in copy-initialization context using list-initialization syntax: std::tuple<int, int> foo_tuple() { // return {1, -1}; // Error before N4387 return std::make_tuple(1, -1); // Always works } Note that if some element of the list is not implicitly convertible to the corresponding element of the target tuple, the constructors become explicit: using namespace std::chrono; void launch_rocket_at(std::tuple<hours, minutes, seconds>); launch_rocket_at({hours(1), minutes(2), seconds(3)}); // OK launch_rocket_at({1, 2, 3}); // Error: int is not implicitly convert- ible to duration launch_rocket_at(std::tuple<hours, minutes, seconds>{1, 2, 3}); // OK Example // Run this code #include <iomanip> #include <iostream> #include <memory> #include <string> #include <string_view> #include <tuple> #include <type_traits> #include <vector> // helper function to print a vector to a stream template<class Os, class T> Os& operator<<(Os& os, std::vector<T> const& v) { os << '{'; for (auto i{v.size()}; const T& e : v) os << e << (--i ? "," : ""); return os << '}'; } template<class T> void print_single(T const& v) { if constexpr (std::is_same_v<T, std::decay_t<std::string>>) std::cout << std::quoted(v); else if constexpr (std::is_same_v<std::decay_t<T>, char>) std::cout << "'" << v << "'"; else std::cout << v; } // helper function to print a tuple of any size template<class Tuple, std::size_t N> struct TuplePrinter { static void print(const Tuple& t) { TuplePrinter<Tuple, N - 1>::print(t); std::cout << ", "; print_single(std::get<N - 1>(t)); } }; template<class Tuple> struct TuplePrinter<Tuple, 1> { static void print(const Tuple& t) { print_single(std::get<0>(t)); } }; template<class... Args> void print(std::string_view message, const std::tuple<Args...>& t) { std::cout << message << " ("; TuplePrinter<decltype(t), sizeof...(Args)>::print(t); std::cout << ")\n"; } // end helper function int main() { std::tuple<int, std::string, double> t1; print("Value-initialized, t1:", t1); std::tuple<int, std::string, double> t2{42, "Test", -3.14}; print("Initialized with values, t2:", t2); std::tuple<char, std::string, int> t3{t2}; print("Implicitly converted, t3:", t3); std::tuple<int, double> t4{std::make_pair(42, 3.14)}; print("Constructed from a pair, t4:", t4); // given Allocator my_alloc with a single-argument constructor // my_alloc(int); use my_alloc(1) to allocate 5 ints in a vector using my_alloc = std::allocator<int>; std::vector<int, my_alloc> v{5, 1, my_alloc{/* 1 */}}; // use my_alloc(2) to allocate 5 ints in a vector in a tuple std::tuple<int, std::vector<int, my_alloc>, double> t5 {std::allocator_arg, my_alloc{/* 2 */}, 42, v, -3.14}; print("Constructed with allocator, t5:", t5); } Possible output: Value-initialized, t1: (0, "", 0) Initialized with values, t2: (42, "Test", -3.14) Implicitly converted, t3: ('*', "Test", -3) Constructed from a pair, t4: (42, 3.14) Constructed with allocator, t5: (42, {1,1,1,1,1}, -3.14) 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 LWG 2510 C++11 default constructor was implicit made condi- tionally-explicit constructor of 1-tuple might LWG 3121 C++11 recursively check the constraints; furtherly constrained allocator_arg_t argument brought the con- structor ambiguity the uses-allocator constructor LWG 3158 C++11 corresponding made condi- tionally-explicit to default constructor was implicit LWG 3211 C++11 whether the default constructor of require to be trivial tuple<> is trivial was unspecified N4387 C++11 some constructors were explicit, most con- structors made preventing useful behavior condition- ally-explicit tuple-like constructor may LWG 4045 C++23 potentially create dangling made de- fined as deleted references See also operator= assigns the contents of one tuple to another (public member function) make_tuple creates a tuple object of the type defined by the argument types (C++11) (function template) tie creates a tuple of lvalue references or unpacks a tuple into (C++11) individual objects (function template) forward_as_tuple creates a tuple of forwarding references (C++11) (function template) constructor constructs new pair (public member function of std::pair<T1,T2>) http://cppreference.com 2024.06.10 std::tuple::tuple(3)
NAME | Synopsis | Parameters | Notes | Example | Possible output: | See also
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