17 Concepts library [concepts]

17.5 Comparison concepts [concepts.compare]

17.5.3 Concept EqualityComparable [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;
      { t != u } -> Boolean;
      { u == t } -> Boolean;
      { u != t } -> Boolean;
    };

Let t and u be lvalues of types const remove_reference_t<T> and const remove_reference_t<U> respectively.

weakly-equality-comparable-with<T, U> is satisfied 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 EqualityComparable = weakly-equality-comparable-with<T, T>;

Let a and b be objects of type T.

EqualityComparable<T> is satisfied only if bool(a == b) is true when a is equal to b ([concepts.equality]), and false otherwise.

[ Note

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

— end note

]

🔗

template<class T, class U>
  concept EqualityComparableWith =
    EqualityComparable<T> && EqualityComparable<U> &&
    CommonReference<const remove_reference_t<T>&, const remove_reference_t<U>&> &&
    EqualityComparable<
      common_reference_t<
        const remove_reference_t<T>&,
        const remove_reference_t<U>&>> &&
    weakly-equality-comparable-with<T, 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>&>

EqualityComparableWith<T, U> is satisfied only if bool(t == u) == bool(C(t) == C(u)).