Object Oriented Programming Development - Polymorphism I

1. By: Marc Conrad & Rob Manton University of Luton Email: Marc.Conrad@luton.ac.uk Rob.Manton@luton.ac.uk Room: D104 Object Oriented ProgrammingDevelopment - Polymorphism I

2. Introduction Non object oriented basics Classes Inheritance Aggregation Polymorphism Multifile Development Module Outline

3. Today’s lecture • Polymorphism I • method overloading • operator overloading

4. The Meaning of the word. • From the Greek: • Polus + Morphe = Polumorphos (many ) (shape/form) • The English word "polymorphe" dates from the 19th century and was applied to different animal forms arising in the the same species.

5. The Meaning of the word. • In object-oriented computing it means: different forms of data being handled by the same type of process. • Example: The operator + has a different meaning in the expression 2 + 3 (add two integers) than in 1.7 + 3.3 (add two floating point numbers)

6. Types of Polymorphism • In Object Oriented Programming there are three types of polymorphism: • method overloading, with the special and important case of operator overloading • method overriding • run-time polymorphism

7. In Object Oriented Programming there are three types of polymorphism: method overloading, with the special and important case of operator overloading method overriding run-time polymorphism Method overloading can also be applied in non-object oriented contexts and refers both to functions and methods. Types of Polymorphism

8. In Object Oriented Programming there are three types of polymorphism: method overloading, with the special and important case of operator overloading method overriding run-time polymorphism Method overriding and run-time polymorphism are specific to inheritance hierarchies and object oriented programming (more about this next week..) Types of Polymorphism

9. In Object Oriented Programming there are three types of polymorphism: method overloading, with the special and important case of operator overloading method overriding run-time polymorphism Run-time polymorphism, also called dynamic binding, or late binding is often considered as the object oriented feature of C++. Types of Polymorphism

10. Method & Function Overloading • Overloading a function simply means, that a function is not only defined by its name but by its name and parameter types. • The following functions are different in C++: • int makeBreakfast(int i, int k); • void makeBreakfast(Creature who); • float makeBreakfast();

11. class Creature { private: int yearOfBirth; public: void setYearOfBirth(int year) { yearOfBirth = year; } void setYearOfBirth(Creature other) { yearOfBirth = other.yearOfBirth; } int getYearOfBirth() { return yearOfBirth; } }; Example: The Creature class born1997

12. class Creature { private: int yearOfBirth; public: void setYearOfBirth(int year) { yearOfBirth = year; } void setYearOfBirth(Creature other) { yearOfBirth = other.yearOfBirth; } int getYearOfBirth() { return yearOfBirth; } }; Example: The Creature class These two methods are different. born1997

13. class Creature { private: int yearOfBirth; public: void setYearOfBirth(int year) { yearOfBirth = year; } void setYearOfBirth(Creature other) { yearOfBirth = other.yearOfBirth; } int getYearOfBirth() { return yearOfBirth; } }; Example: The Creature class These two methods are different because they have different argument types. born1997

14. Operator Overloading - Motivation • Question: How many function calls are involved in the following statement? a = 2 + 3

15. Operator Overloading - Motivation • Question: How many function calls are involved in the following statement? a = 2 + 3 • There are two functions implicitly involved:+ and =. • Look at this statement as “assign(a, add(2,3));”

16. Operator Overloading • So, operators as +, -, *, <<, =, etc. can be seen as “functions” as well. That means we can overload operators. • The C++ syntax uses “function names” prefixed with “operator” for overloading operators.

17. class BLT { public: bool bacon; float lettuce; int tomatoes; // Constructor: BLT(bool b, float l, int t); // … (more code) }; A Sandwich filling. may contain bacon (yes/no). a fraction of a lettuce-leaf. a number of tomato slices. Overloading Operators - Example

18. class BLT { public: bool bacon; float lettuce; int tomatoes; // Constructor: BLT(bool b, float l, int t); // … (more code) }; BLT filling1(true,0.5,2); BLT filling2(false,0.2,0); ... BLT filling3 = filling1 + filling2; ... /* Should give a filling with bacon, 0.7 lettuce and 2 tomatoes*/ Overloading Operators - Example

19. class BLT { public: bool bacon; float lettuce; int tomatoes; // Constructor: BLT(bool b, float l, int t); // … (more code) }; BLT filling1(true,0.5,2); BLT filling2(false,0.2,0); … BLT filling3 = filling1 + filling2; ... /* Should give a filling with 3 bacon slices, 0.7 lettuce and 2 tomatoes */ Overloading Operators - Example This is the operator we want to overload

20. Operator Overloading - Example If we try adding the two objects together at the moment we get the expected error message

21. class BLT { public: bool bacon; float lettuce; int tomatoes; // Constructor: BLT(bool b, float l, int t); // … (more code) }; // The C++ Syntax BLT operator+(BLT x, BLT y) { bool b =x.bacon || y.bacon; float l = x.lettuce + y.lettuce; int t = x.tomatoes = y.tomatoes; BLT result(b,l,t); return result; } Overloading Operators - Example

22. Overloading Operators - Example + operator overloaded to accept two BLT objects as arguments.

23. Overloading Operators - Example Note: return type is BLT

24. Overloading Operators - Example Because overloaded + operator returns a BLT object, this works!

25. void operator+=(BLT &x, BLT y) { bool bacon =( x.get_bacon() float lettuce =x.get_lettuce() + int toms=x.get_tomato_slices()+ x.set_bacon(bacon); x.set_lettuce(lettuce); x.set_tomato_slices(toms); } Overloading Operators - Example Where an operator like += actually needs to change the first operand, we need to use the & (call by reference) syntax

26. Operator Overloading • Operators can also be overloaded as methods, e.g. the operator +=: • class BLT { • // … • BLT operator+=(BLT other) { • bacon =( bacon || other.bacon); • tomatoes += other.tomatoes; • lettuce += other.lettuce; • } • //…

27. Operator Overloading += operator overloaded to accept one extra BLT object as an argument -note void return type

28. Operator Overloading The const keyword indicates to the compiler that you are not going to change the other BLT object in any way

29. BLT filling1(true,0.5,2); BLT filling2(false,0.2,0); … filling1 += filling2; ... /* Should give a filling with bacon, 0.7 lettuce and 2 tomatoes*/ Operator Overloading • Operators can also be overloaded as methods, e.g. the operator +=: • class BLT { • // … • BLT operator+=(BLT other) { • bacon =( bacon || other.bacon); • tomatoes += other.tomatoes; • lettuce += other.lettuce; • } • //…

30. Operator Overloading • Operators can also have other types as parameter: • class BLT { • // … • BLT operator*=(int factor) { • tomatoes *= factor; • lettuce *= factor; • } • //…

31. BLT filling1(false,0.5,2); … filling1 *= 2; ... /* Should give a filling with no bacon, 1 lettuce and 4 tomatoes */ Operator Overloading • Operators can also have other types as parameters: • class BLT { • // … • BLT operator*=(int factor) { • tomatoes *= factor; • lettuce *= factor; • } • //…

32. Operator Overloading • The following operators can be overloaded: • new, delete, +, -, *, /, %, ^, &, |, ~, !, =, <, >, +=, -=, *=, /=, %=, ^=, &=, |=, <<, >>, >>=, <<=, ==, !=, <=, >=, &&, ||, ++, --, , , ->*. ->, (), [] • Note that "=" has already a default behaviour. When "overloaded" it will be in fact overridden.

33. Operator Overloading - Interesting Observation • cout << “Hello World\n”; Overloaded << operator

34. Operator Overloading - Interesting Observation The << operator is overloaded to take a BLT object as an argument

35. Operator Overloading - Interesting Observation • BLT myFilling(1,0.5,4); • cout << myFilling << endl; Now we can perform class-specific output using the standard << syntax!

36. Operator Overloading - Summary • Operators may be overloaded to work with user defined data types (objects). • The syntax for overloading involves the 'operator' keyword and the operator. • Note: In a good design it is important, that the normal meanings of operators are not distorted (don't subtract with a + operator)