Consider overloading unary operator ! to test whether an object of your own String class is empty. Such a function would return a bool result. When a unary operator such as ! is overloaded as a member function with no arguments and the compiler sees the expression !s (in which s is an object of class String), the compiler generates the function call s.operator!(). The operand s is the String object for which the String class member function operator! is being invoked. The function is declared as follows:

class String {
public:
   bool operator!() const;
   ...
};

A unary operator such as ! may be overloaded as a non-member function with one parameter. If s is a String class object (or a reference to a String class object), then !s is treated as if the call operator!(s) had been written, invoking the non-member operator! function that’s declared as follows:

bool operator!(const String&);