[concept.assignable]

🔗

template<class LHS, class RHS>
  concept assignable_from =
    is_lvalue_reference_v<LHS> &&
    common_reference_with<const remove_reference_t<LHS>&, const remove_reference_t<RHS>&> &&
    requires(LHS lhs, RHS&& rhs) {
      { lhs = std::forward<RHS>(rhs) } -> same_as<LHS>;
    };

Let:

(1.1)
lhs be an lvalue that refers to an object lcopy such that decltype((lhs)) is LHS,
(1.2)
rhs be an expression such that decltype((rhs)) is RHS, and
(1.3)
rcopy be a distinct object that is equal to rhs.

LHS and RHS model assignable_from<LHS, RHS> only if

(1.4)
addressof(lhs = rhs) == addressof(lcopy).
(1.5)
After evaluating lhs = rhs:
- (1.5.1)
  lhs is equal to rcopy, unless rhs is a non-const xvalue that refers to lcopy.
- (1.5.2)
  If rhs is a non-const xvalue, the resulting state of the object to which it refers is valid but unspecified ([lib.types.movedfrom]).
- (1.5.3)
  Otherwise, if rhs is a glvalue, the object to which it refers is not modified.

[Note 1:

Assignment need not be a total function ([structure.requirements]); in particular, if assignment to an object x can result in a modification of some other object y, then x = y is likely not in the domain of =.

— end note]

18 Concepts library [concepts]

18.4 Language-related concepts [concepts.lang]

18.4.8 Concept assignable_from [concept.assignable]