C++ named requirements: EqualityComparable
The type must work with == operator and the result should have standard semantics.
T satisfies EqualityComparable if
c, expressions of type
The following expressions must be valid and have their specified effects
||implicitly convertible to bool|| Establishes an equivalence relation, that is, it satisfies the following properties:
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).