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std::variant::variant

constexpr variant() noexcept(/* see below */);
 (1) (since C++17) 
variant(const variant& other);
 (2) (since C++17) 
variant(variant&& other) noexcept(/* see below */);
 (3) (since C++17) 
template< class T >
constexpr variant(T&& t) noexcept(/* see below */);
 (4) (since C++17) 
template< class T, class... Args >
constexpr explicit variant(std::in_place_type_t<T>, Args&&... args);
 (5) (since C++17) 
template< class T, class U, class... Args >
constexpr explicit variant(std::in_place_type_t<T>,
                           std::initializer_list<U> il, Args&&... args);
 (6) (since C++17) 
template< std::size_t I, class... Args >
constexpr explicit variant(std::in_place_index_t<I>, Args&&... args);
 (7) (since C++17) 
template <size_t I, class U, class... Args>
constexpr explicit variant(std::in_place_index_t<I>,
                           std::initializer_list<U> il, Args&&... args);
 (8) (since C++17) 
template <class Alloc>
variant(std::allocator_arg_t, const Alloc& a);
 (9) (since C++17) 
template <class Alloc>
variant(std::allocator_arg_t, const Alloc& a, const variant& other);
 (10) (since C++17) 
template <class Alloc>
variant(std::allocator_arg_t, const Alloc& a, variant&& other);
 (11) (since C++17) 
template <class Alloc, class T>
variant(std::allocator_arg_t, const Alloc& a, T&& t);
 (12) (since C++17) 
template <class Alloc, class T, class... Args>
variant(std::allocator_arg_t, const Alloc& a,
        std::in_place_type_t<T>, Args&&... args);
 (13) (since C++17) 
template <class Alloc, class T, class U, class... Args>
variant(std::allocator_arg_t, const Alloc& a,
        std::in_place_type_t<T>, std::initializer_list<U> il, Args&&... args);
 (14) (since C++17) 
template <class Alloc, size_t I, class... Args>
variant(std::allocator_arg_t, const Alloc& a,
        std::in_place_index_t<I>, Args&&... args);
 (15) (since C++17) 
template <class Alloc, size_t I, class U, class... Args>
variant(std::allocator_arg_t, const Alloc& a,
        std::in_place_index_t<I>, std::initializer_list<U> il, Args&&... args);
 (16) (since C++17)


Constructs a new variant object.

1) Default constructor. Constructs a variant holding the value-initialized value of the first alternative (index() is zero). This constructor is constexpr if and only if the value initialization of the alternative type T_0 would satisfy the requirements for a constexpr function. This overload only participates in overload resolution if std::is_default_constructible_v<T_0> is true.
2) Copy constructor. If other is not valueless_by_exception, constructs a variant holding the same alternative as other and direct-initializes the contained value with std::get<other.index()>(other). Otherwise, initializes a valueless_by_exception variant. This overload only participates in overload resolution if std::is_copy_constructible_v<T_i> is true for all T_i in Types....
3) Move constructor. If other is not valueless_by_exception, constructs a variant holding the same alternative as other and direct-initializes the contained value with std::get<other.index()>(std::move(other)). Otherwise, initializes a valueless_by_exception variant. This overload only participates in overload resolution if std::is_move_constructible_v<T_i> is true for all T_i in Types...
4) Converting constructor. Constructs a variant holding the alternative 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. Direct-initializes the contained value as if by direct non-list-initialization from std::forward<T>(t). This overload only participates in overload resolution if std::is_same_v<std::decay_t<T>, variant> is false, std::decay_t<T> is neither 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. 
variant<string> v("abc"); // OK
variant<string, string> w("abc"); // ill-formed, can't select the alternative to convert to
variant<string, bool> x("abc"); // OK, but chooses bool
5) Constructs a variant with the specified alternative T and initializes 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 only participates in overload resolution 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 initializes 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 only participates in overload resolution if there is exactly one occurrence of T in Types... and std::is_constructible_v<T, initializer_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::forward<Args>(args).... If T_i's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. This overload only participates in overload resolution 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::forward<Args>(args).... If T_i's selected constructor is a constexpr constructor, this constructor is also a constexpr constructor. This overload only participates in overload resolution if I < sizeof...(Types) and std::is_constructible_v<T_i, std::initializer_list<U>&, Args...> is true.
9-16) Same as (1-8), except the contained value is constructed following the uses-allocator protocol

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
a - allocator to pass to the contained value
Type requirements
-Alloc must meet the requirements of Allocator in order to use overloads (9).

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 selected 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 constructor of the selected alternative

Example

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