18 Concepts library [concepts]

18.5 Comparison concepts [concepts.compare]

18.5.4 Concept equality_comparable [concept.equalitycomparable]

template<class T, class U>
  concept weakly-equality-comparable-with = // exposition only
    requires(const remove_reference_t<T>& t,
             const remove_reference_t<U>& u) {
      { t == u } -> boolean-testable;
      { t != u } -> boolean-testable;
      { u == t } -> boolean-testable;
      { u != t } -> boolean-testable;
    };

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

T and U model weakly-equality-comparable-with<T, U> only if

(1.1)
t == u, u == t, t != u, and u != t have the same domain.
(1.2)
bool(u == t) == bool(t == u).
(1.3)
bool(t != u) == !bool(t == u).
(1.4)
bool(u != t) == bool(t != u).

🔗

template<class T>
  concept equality_comparable = weakly-equality-comparable-with<T, T>;

Let a and b be objects of type T.

T models equality_comparable only if bool(a == b) is true when a is equal to b ([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 =
    equality_comparable<T> && equality_comparable<U> &&
    comparison-common-type-with<T, U> &&
    equality_comparable<
      common_reference_t<
        const remove_reference_t<T>&,
        const remove_reference_t<U>&>> &&
    weakly-equality-comparable-with<T, U>;

Given types T and U, let t and t2 be lvalues denoting distinct equal objects of types const remove_reference_t<T> and remove_cvref_t<T>, respectively, let u and u2 be lvalues denoting distinct equal objects of types const remove_reference_t<U> and remove_cvref_t<U>, respectively, and let C be: common_reference_t<const remove_reference_t<T>&, const remove_reference_t<U>&> T and U model equality_comparable_with<T, U> only if bool(t == u) == bool(CONVERT_TO_LVALUE<C>(t2) == CONVERT_TO_LVALUE<C>(u2))