CS-240

1. CS-240 Operator Overloading Dick Steflik

2. Operator Overloading • What is it? • assigning a new meaning to a specific operator when used in the context of a specific class • ex. << is normally the shift left operator; but when used in conjunction with the class iostream it becomes the operator insertion; this is because the writers of the iostream class overloaded << to be insertion • cout << “some string” << endl; • remember cout is a predefined object of class iostream • in another class, << could be overloaded to mean something.

3. Operator Overloading • Why do we do it? • the c++ developers felt that it made more sense to overload an operator than to come up with some name for a function than meant the same thing as the operator. • using an overloaded operator takes fewer keystrokes • many people feel that an overloaded operator is more self documenting

4. Operator Overloading • Three basic ways: • as a free function (not part of a class) • as a member function • as a friend function

5. Operator Overloading class Rational { Rational (int , int); const Rational & operator = (const Rational & rhs); const Rational & operator + (const Rational & rhs); bool operator == (const Rational & rhs); int getNumer() const { return numer; } int getDenom() const { return denom; } private: int numer; int denom; }

6. Overloading = as a member const Rational & Rational :: operator = (const Rational & rhs) { if (this != &rhs) { numer = rhs.numer; denom = rhs.denom; } return *this } Rational r1 , r2; r1 = r2; -note: the if statement catches the special case where someone is trying r1=r1, the if statement makes the code faster

7. Overloading + as a member const Rational Rational::operator + (const Rational & rhs) { Rational answer ( *this); // initialize the answer with the current object answer += rhs; // add the second operand (assumes += has been overloaded) return answer; // return the answer via the copy constructor }

8. Overloading == as a free function // not defined in the Rational class bool operator == (const & Rational lhs , const & Rational rhs) { return (lhs.getDenom() * rhs.getNumer() == lhs.getNumer() * rhs.getDenom()); } Rational r1 , r2; if ( r1 == r2 ) cout……… else cout

9. Overloading == as a member const bool Rational::operator == (const Rational & rhs ) { return ( numer * rhs.denom == denom * rhs.numer); } Rational r1 , r2 ; if (r1 == r2 ) cout ……. else cout ……

10. Friend Functions • not a member function • has access to private data • member functions work with the current (named object) friend functions work with multiple objects of the same class • tag as a friend of the class • as part of class definition identify, by prototype, each friend of the class

11. Friend Functions (cont.) • Friend functions are needed in C++ due to C++’s flawed object model, Java has a better model (all objects are derived from a single object). • define the prototype in the public section of the class definition • precede the prototype with the keyword “friend”

12. Friend Functions (more) • define the friend implementation in the .cpp file with the member functions • do not precede the function name with the class name and the scoping operator (ex. classname::)

13. Overloading == as a friend class Rational { Rational (int , int); const Rational & operator = (const Rational & rhs); const Rational & operator + (const Rational & rhs); friend bool operator == (const Rational & lhs , const Rational & rhs); private: int numer; int denom; } bool operator == (const Rational & lhs , const Rational & rhs) { return ( lhs.numer * rhs.denom == lhs.denom * rhs.numer); }

14. Overloading << as a friend class Rational { Rational (int , int); const Rational & operator = (const Rational & rhs); const Rational & operator + (const Rational & rhs); friend ostream & operator << (ostream & ostr , const Rational * rhs); private: int numer; int denom; } ostream & operator << (ostream & ostr , const Rational * rhs) { ostr << “numerator = “ << rhs.numer << “ denominator = “ << rhs.denom; }

15. More Overloading Thoughts • = overload as a member function • == != <= >= < > overload as a member • >> << (insertion and extraction) overload as non-members (friends) returning type iostream • +-*/% (arithmetics) overload as members • += -= ... overload same as + and -

16. Please note: • The only operators that cannot be overloaded are • . (dot operator) • .* (pointer-to-member) • sizeof • ?: (three operands)

17. Only existing operators can be overloaded. • new operators cannot be created (have to be made a named function member)