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

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constexpr variant& operator=(const variant& rhs); | (1) | (since C++17)
constexpr variant& operator=(variant&& rhs) noexcept(/* see below */); | (2) | (since C++17)
template <class T> variant& operator=(T&& t) noexcept(/* see below */); | (3) | (since C++17)

Assigns a new value to an existing `variant` object.

1) Copy-assignment:

- If both `*this` and `rhs` are valueless by exception, does nothing.
- Otherwise, if `rhs` is valueless, but `*this` is not, destroys the value contained in `*this` and makes it valueless.
- Otherwise, if `rhs` holds the same alternative as `*this`, assigns the value contained in `rhs` to the value contained in `*this`. If an exception is thrown, `*this` does not become valueless: the value depends on the exception safety guarantee of the alternative's copy assignment.
- Otherwise, if the alternative held by `rhs` is either nothrow copy constructible or not nothrow move constructible (as determined by std::is_nothrow_copy_constructible and std::is_nothrow_move_constructible, respectively), equivalent to this->emplace<rhs.index()>(get<rhs.index()>(rhs)).
- Otherwise, equivalent to this->operator=(variant(rhs)). Note that `*this` may become `valueless_by_exception` as described in (2).

This overload is defined as deleted unless std::is_copy_constructible_v<T_i> and std::is_copy_assignable_v<T_i> are both `true` for all `T_i` in `Types...`. This overload is trivial if std::is_trivially_copy_constructible_v<T_i>,std::is_trivially_copy_assignable_v<T_i> and std::is_trivially_destructible_v<T_i> are all `true` for all `T_i` in `Types...`.
2) Move-assignment:

- If both `*this` and `rhs` are valueless by exception, does nothing
- Otherwise, if `rhs` is valueless, but `*this` is not, destroys the value contained in `*this` and makes it valueless
- Otherwise, if `rhs` holds the same alternative as `*this`, assigns std::get<j>(std::move(rhs)) to the value contained in `*this`, with `j` being `index()`. If an exception is thrown, `*this` does not become valueless: the value depends on the exception safety guarantee of the alternative's move assignment.
- Otherwise (if `rhs` and `*this` hold different alternatives), equivalent to this->emplace<rhs.index()>(get<rhs.index()>(std::move(rhs))). If an exception is thrown by `T_i`'s move constructor, `*this` becomes `valueless_by_exception`.

This overload only participates in overload resolution if std::is_move_constructible_v<T_i> and std::is_move_assignable_v<T_i> are both `true` for all `T_i` in `Types...`. This overload is trivial if std::is_trivially_move_constructible_v<T_i>, std::is_trivially_move_assignable_v<T_i>, and std::is_trivially_destructible_v<T_i> are all `true` for all `T_i` in `Types...`.
3) Converting assignment.

- Determines 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.

- If `*this` already holds a `T_j`, assigns std::forward<T>(t) to the value contained in `*this`. If an exception is thrown, `*this` does not become valueless: the value depends on the exception safety guarantee of the assignment called.
- Otherwise, if std::is_nothrow_constructible_v<T_j, T> || !std::is_nothrow_move_constructible_v<T_j> is true, equivalent to this->emplace<j>(std::forward<T>(t));
- Otherwise, equivalent to this->operator=(variant(std::forward<T>(t))).

This overload only participates in overload resolution if std::decay_t<T> (until C++20)std::remove_cvref_t<T> (since C++20) is not the same type as variant and `std::is_assignable_v<T_j&, T>` is true and `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.

    std::variant<string> v1;
    v1 = "abc"; // OK
    std::variant<std::string, std::string> v2;
    v2 = "abc"; // Error
    std::variant <std::string, bool> v3;
    v3 = "abc"; // OK, chooses string; bool is not a candidate
    std::variant<float, long, double> v4; //holds float
    v4 = 0; // OK, holds long; float and double are not candidates

### Parameters

rhs | - | another `variant`
t | - | a value convertible to one of the variant's alternatives

### Return value

*this

### Exceptions

1) May throw any exception thrown by assignment and copy/move initialization of any alternative
2)
`noexcept` specification:
noexcept(((std::is_nothrow_move_constructible_v<Types> && std::is_nothrow_move_assignable_v<Types>) && ...))
3)
`noexcept` specification:
noexcept(std::is_nothrow_assignable_v<T_j&, T> && std::is_nothrow_constructible_v<T_j, T>)

### Example

| This section is incomplete Reason: no example

### 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 3024 | C++17 | copy assignment operator doesn't participate in overload resolution if any member type is not copyable | defined as deleted instead
P0602R4 | C++17 | copy/move assignment may not be trivial even if underlying operations are trivial | required to propagate triviality
P0608R3 | C++17 | converting assignment blindly assembles an overload set, leading to unintended conversions | narrowing and boolean conversions not considered

### See also

emplace | constructs a value in the variant, in place (public member function)

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