Overview

1. Overview Class Scope Review: Object parameters passed by value reference constant reference Friend function Overloading operator

2. Object parameters • Objects can be passed by: • Value • Reference • Constant Reference

3. Fraction Class // class declaration class Fraction { private: int num, den; public: Fraction(int=0, int=1); void add(Fraction, Fraction); void reduce(); float decimal(); void reveal(int &, int &); };

4. Object parameters There are three ways to declare the add() member function of the Fraction class: version 1: void add(Fraction, Fraction); version 2: void add(Fraction &, Fraction &); version 3: void add(const Fraction &, const Fraction &);

5. Object parameters Here is part of the main(): Fraction x(1,2); Fraction y(3,4); Fraction z; z.add(x,y); Function invocation same regardless of its declaration

6. Object parameters: Diagram of version 1 when z.add(x,y) executes main() num 1 x den 2 add() num 3 y den 4 num 1 A den 2 z num 0 num 3 den 1 B den 4

7. Object parameters: Explanation of version 1 when z.add(x,y) executes Parameters passed by value A copy of the actual parameters x and y are sent to formal parameters A and B. This is OK for small object but very inefficient for larger objects

8. Object parameters: Diagram of version 2 when z.add(x,y) executes main() num 1 x den 2 add() num 3 y den 4 A z num 0 den 1 B

9. Object parameters: Explanation of version 2 when z.add(x,y) executes Parameters passed by reference More efficient Dangerous: function can inadvertently (or maliciously) change an actual parameter (in the main) by changing the formal parameter. It is not the case here.

10. Object parameters: Diagram of version 3 when z.add(x,y) executes main() num 1 x den 2 add() num 3 y den 4 A z num 0 den 1 B

11. Object parameters: Explanation of version 3 when z.add(x,y) executes Parameters passed by constant reference Both of the best world: security and efficiency No Danger: compiler will not allow to modify the actual parameter, even the formal parameters. If the function tries to do this: • A.num = 1000; //Will not be allowed by the compiler • int i; • i = A.num ; //Will be allowed by the compiler

12. Friend Functions A friend function is not a member function of a class but is allowed to touch anything protected or private It is not a category such as Private or Public

13. Friend or Foe?? Let's look at Fraction again. Consider the Fraction::add() function. It's a __________ function. Can it be restructured as a friend function?

14. Friend Functions To convert Fraction::add() function from a member function to a friend function: 1. put it onto the friends list 2. change its return type 3. remove its class scope operator

15. add() – Member Version declaration: void add(Fraction, Fraction); definition: void Fraction::add(Fraction A, Fraction B) { num = A.num*B.den + B.num*A.den; den = A.den * B.den; }

16. add() – Member Version invocation: Fraction x(1,2), y(3,4), z; z.add(x,y); //now z holds the value 5/4

17. add() – Friend Version declaration: friendFraction add(Fraction, Fraction); definition: Fraction add(Fraction A, Fraction B) { Fraction C; C.num = A.num*B.den +B.num*A.den; C.den = A.den * B.den; return C; }

18. add() – Friend Version invocation: Fraction x(1,2), y(3,4), z; z = add(x,y);

19. Friend Functions Member function example: z.reduce(); Friend function example: z = reduce(z); How would you make reduce() a friend function???

20. Friend Functions Review: Function overloading What does it mean to "overload" a function?   same function name with different argument types   functions must differ by more than return type!

21. Friend Functions An example: a findLowest function char findLowest(char, char, char); int findLowest(int, int, int); float findLowest(float, float, float); double findLowest(double, double, double); long findLowest(long, long, long); BankAccount findLowest(BankAccount,BankAccount,BankAccount); Another example: constructor functions:  Triangle(); Triangle(int, int, int);

22. Friend Functions It's a very short jump from the friend add function to an overloaded operator: Fraction add(Fraction A, Fraction B) { fraction C; C.num = A.num + B.num; C.den = B.den * A.den; C.reduce(); return C; }

23. Friend Functions Fraction operator+ (fraction A, fraction B) { Fraction C; C.num = A.num + B.num; C.den = B.den * A.den; C.reduce(); return C; }

24. Friend Functions Now in main instead of   z = add(x, y) we can say:   z = x + y; How would the declaration of the fraction class need to change? In fraction.h: friend Fraction add(Fraction, Fraction); friend Fraction operator+(Fraction, Fraction);

25. Operator Overloading We can also overload C++ operators. We can make our own = operator: in .h file void operator = (Fraction); in .cpp file void Fraction::operator= (Fraction rightSide) { num = rightSide.num; den = rightSide.den; }

26. Operator Overloading We can make our own = operator: in main Fraction x(1,2); Fraction z; z.operator=(x); //or just z = x;

27. #include <iostream.h> #include "fraction_class.h" void main() { int i, j; Fraction x(1,2); x.reveal(i,j); cout<<"Fraction x is "<<i<<"/"<<j<<endl; Fraction z; z.operator=(x); z.reveal(i,j); cout<<"Fraction z is "<<i<<"/"<<j<<endl; z = x; z.reveal(i,j); cout<<"Fraction z is "<<i<<"/"<<j<<endl; } Fraction x is 1/2 Fraction z is 1/2 Fraction z is 1/2

28. Operator Overloading What happens when Fraction::operator=() is called?

29. Operator Overloading in .h file void operator = (Fraction &); in .cpp file   void Fraction::operator= (Fraction & rightSide) { num = rightSide.num; den = rightSide.den; } What is the danger now??

30. Operator Overloading in .h file void operator = (const Fraction&); in .cpp file void Fraction::operator= (const Fraction & rightSide) { num = rightSide.num; den = rightSide.den; } This is an example of "overloading" an operator.

31. Operator Overloading How would we overload the + operator for fractions?? Two ways: 1) use analogy to the operator= example 2) use a friend function