std::variant::operator=

 

C++

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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

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optional

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in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

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std::variant

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variant::operator=

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visit

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operator==operator!=operator<operator<=operator>operator>=

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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 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 *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::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>)

See also

emplace

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