Operator functions can be called directly, although they are usually invoked indirectly by the use of the overload operator:

c3 = c1.operator + (c2);   // same as c3 = c1 + c2

Apart from new and delete, which have their own rules, an operator function must either be a nonstatic member function or have at least one argument of class type. The operator functions =, ( ), [ ] and -> must be nonstatic member functions. 

Enumerations can have overloaded operators. However, the operator functions =, ( ), [ ], and -> cannot be overloaded for enumerations.