[concept.assignable] (original) (raw)

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](#concept:assignable%5Ffrom "18.4.8 Concept assignable_­from [concept.assignable]") = is_lvalue_reference_v<LHS> && [common_reference_with](concept.commonref#concept:common%5Freference%5Fwith "18.4.5 Concept common_­reference_­with [concept.commonref]")<const remove_reference_t<LHS>&, const remove_reference_t<RHS>&> && requires(LHS lhs, RHS&& rhs) { { lhs = std::forward<RHS>(rhs) } -> [same_as](concept.same#concept:same%5Fas "18.4.2 Concept same_­as [concept.same]")<LHS>;};

Let:

• lhs be an lvalue that refers to an object lcopy such thatdecltype((lhs)) is LHS,
• rhs be an expression such that decltype((rhs)) isRHS, and
• rcopy be a distinct object that is equal to rhs.

LHS and RHS modelassignable_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_]