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std::variant::operator=

From cppreference.com
< cpp‎ | utility‎ | variant
 
 
 
 
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 rhs and *this hold different alternatives)
  • copies the value contained in rhs to a temporary. If an exception is thrown by the copy constructor, *this remains unchanged
  • destroys any value contained in *this
  • sets *this to hold the same alternative as rhs
  • initializes the value contained in *this as if direct-non-list-initializing an object of type T_j with std::forward<T_j>(TMP), with TMP being the temporary and j being rhs.index(). If an exception is thrown by T_j's move constructor, *this becomes valueless_by_exception
This overload only participates in overload resolution if std::is_copy_constructible_v<T_i> && std::is_move_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)
  • destroys any value contained in *this
  • sets *this to hold the same alternative as rhs
  • initializes the value contained in *this as if direct-non-list-initializing an object of type T_j with std::get<j>(std::move(rhs)) with j being rhs.index(). 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,
  • destroys the value contained in *this (if any)
  • sets this to hold the alternative T_j as selected above
  • direct-initializes the contained value as if by direct-non-list-initialization with std::forward<T>(t) as the initializer. If an exception is thrown during this initialization, *this becomes valueless_by_exception

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

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] See also

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