# std::equality_comparable, std::equality_comparable_with

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< cpp‎ | concepts

Defined in header `<concepts>` |  |
template< class T > concept equality_comparable = __WeaklyEqualityComparableWith<T, T>; | (1) | (since C++20)
template< class T, class U > concept equality_comparable_with = std::equality_comparable<T> && std::equality_comparable<U> && std::common_reference_with< const std::remove_reference_t<T>&, const std::remove_reference_t<U>&> && std::equality_comparable< std::common_reference_t< const std::remove_reference_t<T>&, const std::remove_reference_t<U>&>> && __WeaklyEqualityComparableWith<T, U>; | (2) | (since C++20)
template< class T, class U > concept __WeaklyEqualityComparableWith = requires(const std::remove_reference_t<T>& t, const std::remove_reference_t<U>& u) { { t == u } -> boolean-testable; { t != u } -> boolean-testable; { u == t } -> boolean-testable; { u != t } -> boolean-testable; }; | (3) | (exposition only*)

1) The concept `std::equality_comparable` specifies that the comparison operators `==` and `!=` on `T` reflects equality: `==` yields true if and only if the operands are equal.
2) The concept `std::equality_comparable_with` 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.
3) The exposition-only concept `__WeaklyEqualityComparableWith` 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.

### Semantic requirements

These concepts are modeled only if they are satisfied and all concepts they subsume are modeled.

1) std::equality_comparable<T> is modeled 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.
2) std::equality_comparable_with<T, U> is modeled 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)).
3) __WeaklyEqualityComparableWith<T, U> is modeled 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 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 implicitly requires additional variations of that expression that accept a non-constant lvalue or (possibly constant) rvalue for the given operand unless such an expression variation is explicitly required with differing semantics. These implicit expression variations must meet the same semantic requirements of the declared expression. The extent to which an implementation validates the syntax of the variations is unspecified.

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