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std::variant::operator=

From cppreference.com
< cpp‎ | utility‎ | variant
 
 
 
 
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 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> 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<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> 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.

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

Contents

[edit] Parameters

rhs - another variant
t - a value convertible to one of the variant's alternatives

[edit] Return value

*this

[edit] Exceptions

1) May throw any exception thrown by assignment and copy/move initialization of any alternative
2)
noexcept specification:  
3)
noexcept specification:  

[edit] Example

[edit] 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

[edit] See also

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