The type must work with == operator and the result should have standard semantics.
c, expressions of type
The following expressions must be valid and have their specified effects
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
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
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).