# C++ concepts: EqualityComparable

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< cpp‎ | concept

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 `const T`

The following expressions must be valid and have their specified effects

Expression | Return type | Requirements
--- | --- | ---

`a == b`

implicitly convertible to bool

Establishes equivalence relation with 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).

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