[concept.equalitycomparable] (original) (raw)

18 Concepts library [concepts]

18.5 Comparison concepts [concepts.compare]

18.5.3 Concept equality_­comparable [concept.equalitycomparable]

Given types T and U, let t and u be lvalues of typesconst remove_­reference_­t<T> andconst remove_­reference_­t<U> respectively.

T and U model_weakly-equality-comparable-with_<T, U> only if

• t == u, u == t, t != u, and u != t have the same domain.
• bool(u == t) == bool(t == u).
• bool(t != u) == !bool(t == u).
• bool(u != t) == bool(t != u).

template<class T> concept [equality_­comparable](#concept:equality%5Fcomparable "18.5.3 Concept equality_­comparable [concept.equalitycomparable]") = [_weakly-equality-comparable-with_](#concept:weakly-equality-comparable-with "18.5.3 Concept equality_­comparable [concept.equalitycomparable]")<T, T>;

Let a and b be objects of type T.

T models equality_­comparable only ifbool(a == b) is true when a is equal tob ([concepts.equality]), and false otherwise.

[Note 1:

The requirement that the expression a == b is equality-preserving implies that == is transitive and symmetric.

— _end note_]

template<class T, class U> concept [equality_­comparable_­with](#concept:equality%5Fcomparable%5Fwith "18.5.3 Concept equality_­comparable [concept.equalitycomparable]") = equality_comparable<T> && equality_comparable<U> && common_reference_with<const remove_reference_t<T>&, const remove_reference_t<U>&> && equality_comparable< common_reference_t< const remove_reference_t<T>&,const remove_reference_t<U>&>> && [_weakly-equality-comparable-with_](#concept:weakly-equality-comparable-with "18.5.3 Concept equality_­comparable [concept.equalitycomparable]")<T, U>;

Given types T and U, let t be an lvalue of type const remove_­reference_­t<T>,u be an lvalue of type const remove_­reference_­t<U>, and C be:common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&> T and U modelequality_­comparable_­with<T, U> only ifbool(t == u) == bool(C(t) == C(u)).