Difference between revisions of "cpp/concepts/equality comparable"
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const std::remove_reference_t<T>&, | const std::remove_reference_t<T>&, | ||
const std::remove_reference_t<U>&>> && | const std::remove_reference_t<U>&>> && | ||
| − | + | __WeaklyEqualityComparableWith<T, U>; | |
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Revision as of 05:42, 29 August 2019
| Defined in header <concepts>
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| template<class T, class U> concept __WeaklyEqualityComparableWith = // exposition only |
(1) | |
| template < class T > concept equality_comparable = __WeaklyEqualityComparableWith<T, T>; |
(2) | (since C++20) |
| template <class T, class U> concept equality_comparable_with = |
(3) | (since C++20) |
__WeaklyEqualityComparableWith<T, U> specifies that an object of type T and an object of type U can be compared for equality with each other (in either order) using both == and !=, and the results of the comparisons are consistent. __WeaklyEqualityComparableWith<T, U> is satisfied only if given
-
t, an lvalue of type const std::remove_reference_t<T> and -
u, an lvalue of type const std::remove_reference_t<U>,
the following are true:
- t == u, u == t, t != u,u != t have the same domain;
- bool(u == t) == bool(t == u);
- bool(t != u) == !bool(t == u); and
- bool(u != t) == bool(t != u).
equality_comparable<T> specifies that the comparison operators == and != on T reflects equality: == yields true if and only if the operands are equal.equality_comparable<T> is satisfied only if, given objects a and b of type T, bool(a == b) is true if and only if a and b are equal. Together with the requirement that a == b is equality preserving, this implies that == is symmetric and transitive, and further that == is reflexive for all objects a that are equal to at least one other object.equality_comparable_with<T, U> specifies that the comparison operators == and != on (possibly mixed) T and U operands yield results consistent with equality. Comparing mixed operands yields results equivalent to comparing the operands converted to their common type.equality_comparable_with<T, U> is satisfied only if, given any lvalue t of type const std::remove_reference_t<T> and any lvalue u of type const std::remove_reference_t<U>, and let C be std::common_reference_t<const std::remove_reference_t<T>&, const std::remove_reference_t<U>&>, bool(t == u) == bool(C(t) == C(u)).
Equality preservation
Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).
Implicit expression variations
A requires expression that uses an expression that is non-modifying for some constant lvalue operand also requires implicit expression variations.
