std::variant::operator= (3) Linux Manual Page
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:
*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 (public member function)