22 General utilities library [utilities]

22.6 Variants [variant]

22.6.3 Class template variant [variant.variant]

22.6.3.4 Assignment [variant.assign]

constexpr variant& operator=(const variant& rhs);
Let j be rhs.index().
Effects:
  • If neither *this nor rhs holds a value, there is no effect.
  • Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
  • Otherwise, if index() == j, assigns the value contained in rhs to the value contained in *this.
  • Otherwise, if either is_nothrow_copy_constructible_v<> is true or is_nothrow_move_constructible_v<> is false, equivalent to emplace<j>(get<j>(rhs)).
  • Otherwise, equivalent to operator=(variant(rhs)).
Postconditions: index() == rhs.index().
Returns: *this.
Remarks: This operator is defined as deleted unless is_copy_constructible_v<> && is_copy_assignable_v<> is true for all i.
If is_trivially_copy_constructible_v<> && is_trivially_copy_assignable_v<> && is_trivially_destructible_v<> is true for all i, this assignment operator is trivial.
constexpr variant& operator=(variant&& rhs) noexcept(see below);
Let j be rhs.index().
Constraints: is_move_constructible_v<> && is_move_assignable_v<> is true for all i.
Effects:
  • If neither *this nor rhs holds a value, there is no effect.
  • Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
  • Otherwise, if index() == j, assigns get<j>(std​::​move(rhs)) to the value contained in *this.
  • Otherwise, equivalent to emplace<j>(get<j>(std​::​move(rhs))).
Returns: *this.
Remarks: If is_trivially_move_constructible_v<> && is_trivially_move_assignable_v<> && is_trivially_destructible_v<> is true for all i, this assignment operator is trivial.
The exception specification is equivalent to is_nothrow_move_constructible_v<> && is_nothrow_move_assignable_v<> for all i.
  • If an exception is thrown during the call to 's move construction (with j being rhs.index()), the variant will hold no value.
  • If an exception is thrown during the call to 's move assignment, the state of the contained value is as defined by the exception safety guarantee of 's move assignment; index() will be j.
template<class T> constexpr variant& operator=(T&& t) noexcept(see below);
Let be a type that is determined as follows: build an imaginary function FUN() for each alternative type for which x[] = {std​::​forward<T>(t)}; is well-formed for some invented variable x.
The overload FUN() selected by overload resolution for the expression FUN(std​::​forward<T>(​t)) defines the alternative which is the type of the contained value after assignment.
Constraints:
  • is_same_v<remove_cvref_t<T>, variant> is false,
  • is_assignable_v<&, T> && is_constructible_v<, T> is true, and
  • the expression FUN(std​::​forward<T>(t)) (with FUN being the above-mentioned set of imaginary functions) is well-formed.
    [Note 1: 
    variant<string, string> v; v = "abc"; is ill-formed, as both alternative types have an equally viable constructor for the argument.
    — end note]
Effects:
  • If *this holds a , assigns std​::​forward<T>(t) to the value contained in *this.
  • Otherwise, if is_nothrow_constructible_v<, T> || !is_nothrow_move_constructible_v<> is true, equivalent to emplace<j>(std​::​forward<T>(t)).
  • Otherwise, equivalent to emplace<j>((std​::​forward<T>(t))).
Postconditions: holds_alternative<>(*this) is true, with selected by the imaginary function overload resolution described above.
Returns: *this.
Remarks: The exception specification is equivalent to: is_nothrow_assignable_v<T&, T> && is_nothrow_constructible_v<T, T>
  • If an exception is thrown during the assignment of std​::​forward<T>(t) to the value contained in *this, the state of the contained value and t are as defined by the exception safety guarantee of the assignment expression; valueless_by_exception() will be false.
  • If an exception is thrown during the initialization of the contained value, the variant object is permitted to not hold a value.