# std::variant<Types...>::operator= (3) - Linux Manuals

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

## NAME

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

## Synopsis

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:

* 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.

## 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 *(public member function)*