# 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

- a, b and c, expressions of type `T` or (since C++11) `const T`.

The following expressions must be valid and have their specified effects:

Expression | Return type | Requirements
--- | --- | ---
a == b | implicitly convertible to bool | 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.

### 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`

### See also

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

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