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std::variant::operator=

variant& operator=(const variant& rhs);
(1) (since C++17)
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 other 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<other.index()>(get<other.index()>(other)).
  • Otherwise, equivalent to this->operator=(variant(rhs)).
This overload is defined as deleted unless std::is_copy_constructible_v<T_i> && std::is_copy_assignable_v<T_i> is 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<other.index()>(get<other.index()>(std::move(other))). 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> && std::is_move_assignable_v<T_i> is 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.
  • 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.

variant<string> v1;
v1 = "abc"; // OK
variant<string, string> v2;
v2 = "abc"; // Error
variant <string, bool> v3;
v3 = "abc"; // OK but chooses bool

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:
3)
noexcept specification:

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

See also

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

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