FreeBSD Manual Pages
std::variant::variant(3) C++ Standard Libary std::variant::variant(3) NAME std::variant::variant - std::variant::variant Synopsis constexpr variant() noexcept(/* see below */); (1) (since C++17) constexpr variant( const variant& other ); (2) (since C++17) constexpr variant( variant&& other ) noexcept(/* see below */); (3) (since C++17) template< class T > (4) (since C++17) constexpr variant( T&& t ) noexcept(/* see below */); template< class T, class... Args > (5) (since C++17) constexpr explicit variant( std::in_place_type_t<T>, Args&&... args ); template< class T, class U, class... Args > (6) (since C++17) constexpr explicit variant( std::in_place_type_t<T>, std::initializer_list<U> il, Args&&... args ); template< std::size_t I, class... Args > (7) (since C++17) constexpr explicit variant( std::in_place_index_t<I>, Args&&... args ); template< std::size_t I, class U, class... Args > (8) (since C++17) constexpr explicit variant( std::in_place_index_t<I>, std::initializer_list<U> il, Args&&... args ); Constructs a new variant object. 1) Default constructor. Constructs a variant holding the value-ini- tialized value of the first alternative (index() is zero). * This constructor is constexpr if and only if the value initial- ization of the alternative type T_0 would satisfy the requirements for a const- expr function. * This overload participates in overload resolution only if std::is_default_constructible_v<T_0> is true. 2) Copy constructor. If other is not valueless_by_exception, con- structs a variant holding the same alternative as other and direct-initializes the contained value with std::get<other.index()>(other). Otherwise, initializes a value- less_by_exception variant. * This constructor is defined as deleted unless std::is_copy_con- structible_v<T_i> is true for all T_i in Types.... * It is trivial if std::is_trivially_copy_constructible_v<T_i> is true for all T_i in Types.... 3) Move constructor. If other is not valueless_by_exception, con- structs a variant holding the same alternative as other and direct-initializes the contained value with std::get<other.index()>(std::move(other)). Otherwise, initial- izes a valueless_by_exception variant. * This overload participates in overload resolution only if std::is_move_constructible_v<T_i> is true for all T_i in Types.... * It is trivial if std::is_trivially_move_constructible_v<T_i> is true for all T_i in Types.... 4) Converting constructor. Constructs a variant holding the alterna- tive type T_j that would be selected by overload resolution for the expression F(std::forward<T>(t)) if there was an overload of imaginary function F(T_i) for every T_i from Types... in scope at the same time, except that: * An overload F(T_i) is only considered if the declara- tion T_i x[] = { std::forward<T>(t) }; is valid for some invented vari- able x; Direct-initializes the contained value as if by direct non-list-ini- tialization from std::forward<T>(t). * This overload participates in overload resolution only if * sizeof...(Types) > 0, * std::decay_t<U> (until C++20) std::remove_cvref_t<U> (since C++20) is neither the same type as variant, nor a specialization of std::in_place_type_t, nor a specialization of std::in_place_index_t, * std::is_constructible_v<T_j, T> is true, * and the expression F(std::forward<T>(t)) (with F being the above-mentioned set of imaginary functions) is well formed. * This constructor is a constexpr constructor if T_j's selected constructor is a constexpr constructor. std::variant<std::string> v("abc"); // OK std::variant<std::string, std::string> w("abc"); // ill-formed std::variant<std::string, const char*> x("abc"); // OK, chooses const char* std::variant<std::string, bool> y("abc"); // OK, chooses string; bool is not a candidate std::variant<float, long, double> z = 0; // OK, holds long // float and double are not candidates 5) Constructs a variant with the specified alternative T and ini- tializes the contained value with the arguments std::forward<Args>(args).... * If T's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. * This overload participates in overload resolution only if there is exactly one occurrence of T in Types... and std::is_constructible_v<T, Args...> is true. 6) Constructs a variant with the specified alternative T and ini- tializes the contained value with the arguments il, std::forward<Args>(args).... * If T's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. * This overload participates in overload resolution only if there is exactly one occurrence of T in Types... and std::is_constructible_v<T, ini- tializer_list<U>&, Args...> is true. 7) Constructs a variant with the alternative T_i specified by the index I and initializes the contained value with the arguments std::for- ward<Args>(args).... * If T_i's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. * This overload participates in overload resolution only if I < sizeof...(Types) and std::is_constructible_v<T_i, Args...> is true. 8) Constructs a variant with the alternative T_i specified by the index I and initializes the contained value with the arguments il, std::for- ward<Args>(args).... * If T_i's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. * This overload participates in overload resolution only if I < sizeof...(Types) and std::is_constructible_v<T_i, std::initializer_list<U>&, Args...> is true. Parameters other - another variant object whose contained value to copy/move t - value to initialize the contained value with args... - arguments to initialize the contained value with il - initializer list to initialize the contained value with Exceptions 1) May throw any exception thrown by the value initialization of the first alternative. noexcept specification: noexcept(std::is_nothrow_default_constructible_v<T_0>) 2) May throw any exception thrown by direct-initializing any T_i in Types... 3) May throw any exception thrown by move-constructing any T_i in Types.... noexcept specification: noexcept( (std::is_nothrow_move_constructible_v<Types> && ...)) 4) May throw any exception thrown by the initialization of the se- lected alternative T_j. noexcept specification: noexcept(std::is_nothrow_constructible_v<T_j, T>) 5-8) May throw any exception thrown by calling the selected con- structor of the selected alternative Example // Run this code #include <cassert> #include <iostream> #include <string> #include <variant> #include <vector> template <class Os> Os& operator<< (Os& os, const std::vector<int>& v) { os << "{ "; for (int e: v) { std::cout << e << ' '; } return os << "}"; } int main() { { std::variant<int, std::string> var; // value-initializes first alternative assert(std::holds_alternative<int>(var) && var.index() == 0 && std::get<int>(var) == 0); } { std::variant<std::string, int> var{"STR"}; // initializes first alternative with std::string{"STR"}; assert(var.index() == 0); std::cout << "1) " << std::get<std::string>(var) << '\n'; } { std::variant<std::string, int> var{42}; // initializes second alternative with int = 42; assert(std::holds_alternative<int>(var)); std::cout << "2) " << std::get<int>(var) << '\n'; } { std::variant<std::string, std::vector<int>, float> var{ std::in_place_type<std::string>, 4, 'A'}; // initializes first alternative with std::string{4, 'A'}; assert(var.index() == 0); std::cout << "3) " << std::get<std::string>(var) << '\n'; } { std::variant<std::string, std::vector<int>, char> var{ std::in_place_type<std::vector<int>>, {1,2,3,4,5} }; // initializes second alternative with std::vec- tor{1,2,3,4,5}; assert(var.index() == 1); std::cout << "4) " << std::get<std::vector<int>>(var) << '\n'; } { std::variant<std::string, std::vector<int>, bool> var{ std::in_place_index<0>, "ABCDE", 3}; // initializes first alternative with std::string{"ABCDE", 3}; assert(var.index() == 0); std::cout << "5) " << std::get<std::string>(var) << '\n'; } { std::variant<std::string, std::vector<int>, char> var{ std::in_place_index<1>, 4, 42}; // initializes second alternative with std::vector(4, 42); assert(std::holds_alternative<std::vector<int>>(var)); std::cout << "6) " << std::get<std::vector<int>>(var) << '\n'; } } Output: 1) STR 2) 42 3) AAAA 4) { 1 2 3 4 5 } 5) ABC 6) { 42 42 42 42 } 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 allocator-aware constructors LWG 2901 C++17 provided but construc- tors removed variant can't properly support allocators converting constructor template P0739R0 C++17 interacts constraint added poorly with class template argument deduction copy constructor doesn't participate LWG 3024 C++17 in defined as deleted instead overload resolution if any member type is not copyable copy/move constructors may not be required to propagate P0602R4 C++17 trivial even if underlying triviality constructors are trivial converting constructor blindly P0608R3 C++17 assembles narrowing and boolean an overload set, leading to conversions not considered unintended conversions Pointer to bool conversion converting constructor for bool did is narrow- ing and P1957R2 C++17 not allow converting constructor has implicit conversion no exception for bool http://cppreference.com 2022.07.31 std::variant::variant(3)
NAME | Synopsis | Parameters | Exceptions | Example | Output:
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=std::variant::variant&sektion=3&manpath=FreeBSD+Ports+15.0>