[concept.assignable]
18 Concepts library [concepts]
18.4 Language-related concepts [concepts.lang]
18.4.8 Concept assignable_from [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:
- lhs be an lvalue that refers to an object lcopy such that decltype((lhs)) is LHS,
- rhs be an expression such that decltype((rhs)) is RHS, and
- rcopy be a distinct object that is equal to rhs.
LHS and RHS model assignable_from<LHS, RHS> only if
addressof(lhs = rhs) == addressof(lcopy).
After evaluating lhs = rhs:
lhs is equal to rcopy, unless rhs is a non-const xvalue that refers to lcopy.
If rhs is a non-const xvalue, the resulting state of the object to which it refers is valid but unspecified ([lib.types.movedfrom]).
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]