std::variant::operator=
From cppreference.com
| 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
*thisandrhsare valueless by exception, does nothing. - Otherwise, if
rhsis valueless, but*thisis not, destroys the value contained in*thisand makes it valueless. - Otherwise, if
rhsholds the same alternative as*this, assigns the value contained inrhsto the value contained in*this. If an exception is thrown,*thisdoes not become valueless: the value depends on the exception safety guarantee of the alternative's copy assignment. - Otherwise, if the alternative held by
otheris 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 only participates in overload resolution if
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
*thisandrhsare valueless by exception, does nothing - Otherwise, if
rhsis valueless, but*thisis not, destroys the value contained in*thisand makes it valueless - Otherwise, if
rhsholds the same alternative as*this, assigns std::get<j>(std::move(rhs)) to the value contained in*this, withjbeingindex(). If an exception is thrown,*thisdoes not become valueless: the value depends on the exception safety guarantee of the alternative's move assignment. - Otherwise (if
rhsand*thishold different alternatives), equivalent to this->emplace<other.index()>(get<other.index()>(std::move(other))). If an exception is thrown byT_i's move constructor,*thisbecomes 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_jthat 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 everyT_ifromTypes...in scope at the same time. - If
*thisalready holds aT_j, assigns std::forward<T>(t) to the value contained in*this. If an exception is thrown,*thisdoes 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::is_same_v<std::decay_t<T>, variant> is false 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:
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 |
See also
| constructs a value in the variant, in place (public member function) |