# C++ named requirements: EqualityComparable

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< cpp‎ | named req

The type must work with `==` operator and the result should have standard semantics.

### Requirements

The type `T` satisfies EqualityComparable if given expressions a, b and c of type `T` or (since C++11) const T, the following expression is valid and has its specified effects:

Expression | Type | Effects
--- | --- | ---

a == b

meets BooleanTestable
(until C++20)

Establishes an equivalence relation, that is, it satisfies the following properties:

- For all values of a, a == a yields true.
- If a == b, then b == a.
- If a == b and b == c, then a == c.

models `boolean-testable` (since C++20)

### Notes

To satisfy this requirement, types that do not have built-in comparison operators have to provide a user-defined operator==.

For the types that are both EqualityComparable and LessThanComparable, the C++ standard library makes a distinction between

- Equality, which is the value of the expression a == b and
- Equivalence, which is the value of the expression !(a < b) && !(b < a).

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 283 | C++98 | even if `T` is EqualityComparable, the requirements did not apply to const T objects | they apply to const T instead of `T`
LWG 2114 (P2167R3) | C++98 | convertibility to bool was too weak to reflect the expectation of implementations | requirements strengthened

### See also

equality_comparableequality_comparable_with (C++20) | specifies that operator == is an equivalence relation (concept)

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