# std::variant<Types...>::variant

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< cpp‎ | utility‎ | variant

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 > 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< std::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)

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 constructor is defined as deleted unless std::is_copy_constructible_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, 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...`. 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 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, except that:

  - An overload F(T_i) is only considered if the declaration T_i x[] = { std::forward <T>(t) }; is valid for some invented variable `x`;
  - If `T_i` is (possibly cv-qualified) bool, F(T_i) is only considered if std:remove_cvref_t<T> is also bool.

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 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 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.

### 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 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

### 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 2901 | C++17 | allocator-aware constructors provided but `variant` can't properly support allocators | constructors removed
P0739R0 | C++17 | converting constructor template interacts poorly with class template argument deduction | constraint added
LWG 3024 | C++17 | copy constructor doesn't participate in overload resolution if any member type is not copyable | defined as deleted instead
P0602R4 | C++17 | copy/move constructors may not be trivial even if underlying constructors are trivial | required to propagate triviality
P0608R3 | C++17 | converting constructor blindly assembles an overload set, leading to unintended conversions | narrowing and boolean conversions not considered

### Example

| This section is incomplete Reason: no example

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