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[Page 581]

11.6. Overloading Unary Operators

A unary operator for a class can be overloaded as a non-static member function with no arguments or as a global function with one argument; that argument must be either an object of the class or a reference to an object of the class. Member functions that implement overloaded operators must be non-static so that they can access the non-static data in each object of the class. Remember that static member functions can access only static data members of the class.

Later in this chapter, we will overload unary operator ! to test whether an object of the String class we create (Section 11.10) is empty and return a bool result. Consider the expression !s, in which s is an object of class String. When a unary operator such as ! is overloaded as a member function with no arguments and the compiler sees the expression !s, the compiler generates the call s.operator!(). The operand s is the class object for which the String class member function operator! is being invoked. The function is declared in the class definition as follows:

      class String
         bool operator!() const;
      }; // end class String

A unary operator such as ! may be overloaded as a global function with one argument in two different wayseither with an argument that is an object (this requires a copy of the object, so the side effects of the function are not applied to the original object), or with an argument that is a reference to an object (no copy of the original object is made, so all side effects of this function are applied to the original object). 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 global operator! function that is declared as follows:

      bool operator!( const String & );

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