18 Concepts library [concepts]

18.5 Comparison concepts [concepts.compare]

18.5.3 Concept equality_­comparable [concept.equalitycomparable]

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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; };
1
#
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).
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template<class T> concept equality_­comparable = weakly-equality-comparable-with<T, T>;
2
#
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.
3
#
[Note
:
The requirement that the expression a == b is equality-preserving implies that == is transitive and symmetric.
— end note
]
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template<class T, class U> concept equality_­comparable_­with = 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<T, U>;
4
#
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 model equality_­comparable_­with<T, U> only if bool(t == u) == bool(C(t) == C(u)).