[concept.equalitycomparable] (original) (raw)

18 Concepts library [concepts]

18.5 Comparison concepts [concepts.compare]

18.5.4 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 modelweakly-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 "18.5.4 Concept equality_­comparable [concept.equalitycomparable]") = [_weakly-equality-comparable-with_](#concept:weakly-equality-comparable-with "18.5.4 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_]

Given types T and U, let t and t2 be lvalues denoting distinct equal objects of types const remove_reference_t<T> andremove_cvref_t<T>, respectively, let u and u2 be lvalues denoting distinct equal objects of types const remove_reference_t<U> andremove_cvref_t<U>, respectively, and let 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(CONVERT_TO_LVALUE<C>(t2) == CONVERT_TO_LVALUE<C>(u2))