Chapter 12 - Object-Oriented Programming: Inheritance

1. Chapter 12 - Object-Oriented Programming: Inheritance Outline12.1 Introduction12.2 Base Classes and Derived Classes12.3 protected Members12.4 Relationship between Base Classes and Derived Classes12.5 Case Study: Three-Level Inheritance Hierarchy12.6 Constructors and Destructors in Derived Classes12.7 “Uses A” and “Knows A” Relationships12.8 public, protected and private Inheritance12.9 Software Engineering with Inheritance

2. 12.1 Introduction • Inheritance • Software reusability • Create new class from existing class • Absorb existing class’s data and behaviors • Enhance with new capabilities • Derived class inherits from base class • Derived class • More specialized group of objects • Behaviors inherited from base class • Can customize • Additional behaviors

3. 12.1 Introduction • Class hierarchy • Direct base class • Inherited explicitly (one level up hierarchy) • Indirect base class • Inherited two or more levels up hierarchy • Single inheritance • Inherits from one base class • Multiple inheritance • Inherits from multiple base classes • Base classes possibly unrelated

4. 12.1 Introduction • Three types of inheritance • public • Every object of derived class also object of base class • Base-class objects not objects of derived classes • Example: All cars vehicles, but not all vehicles cars • Can access non-private members of base class • Derived class can effect change to private base-class members • Through inherited non-private member functions • private • Alternative to composition • protected • Rarely used

5. 12.1 Introduction • Abstraction • Focus on commonalities among objects in system • “is-a” vs. “has-a” • “is-a” • Inheritance • Derived class object treated as base class object • Example: Car is a vehicle • Vehicle properties/behaviors also car properties/behaviors • “has-a” • Composition • Object contains one or more objects of other classes as members • Example: Car has a steering wheel

6. 12.2 Base Classes and Derived Classes • Base classes and derived classes • Object of one class “is an” object of another class • Example: Rectangle is quadrilateral. • Class Rectangle inherits from class Quadrilateral • Quadrilateral: base class • Rectangle: derived class • Base class typically represents larger set of objects than derived classes • Example: • Base class: Vehicle • Cars, trucks, boats, bicycles, … • Derived class: Car • Smaller, more-specific subset of vehicles

7. 12.2 Base Classes and Derived Classes • Inheritance examples

8. 12.2 Base Classes and Derived Classes • Inheritance hierarchy • Inheritance relationships: tree-like hierarchy structure • Each class becomes • Base class • Supply data/behaviors to other classes OR • Derived class • Inherit data/behaviors from other classes

9. CommunityMember Single inheritance Single inheritance Single inheritance Multiple inheritance Employee Student Alumnus Faculty Staff Administrator Teacher AdministratorTeacher Fig. 12.2 Inheritance hierarchy for university CommunityMembers.

10. Shape TwoDimensionalShape ThreeDimensionalShape Circle Square Triangle Sphere Cube Tetrahedron Fig. 12.3 Inheritance hierarchy for Shapes.

11. 12.2 Base Classes and Derived Classes • public inheritance • Specify with: Class TwoDimensionalShape : public Shape • Class TwoDimensionalShape inherits from class Shape • Base class private members • Not accessible directly • Still inherited • Manipulate through inherited member functions • Base class public and protected members • Inherited with original member access • friend functions • Not inherited

12. protected Members • protected access • Intermediate level of protection between public and private • protected members accessible to • Base class members • Base class friends • Derived class members • Derived class friends • Derived-class members • Refer to public and protected members of base class • Simply use member names

13. Relationship between Base Classes and Derived Classes • Base class and derived class relationship • Example: Point/circle inheritance hierarchy • Point • x-y coordinate pair • Circle • x-y coordinate pair • Radius

14. Inheritance Concept class Rectangle{ private: int numVertices; float xCoord, yCoord; public: void set(float x, float y, int nV); float area(); }; Polygon Rectangle Triangle class Polygon{ private: int numVertices; float xCoord, yCoord; public: void set(float x, float y, int nV); }; class Triangle{ private: int numVertices; float xCoord, yCoord; public: void set(float x, float y, int nV); float area(); };

15. Inheritance Concept class Polygon{ protected: int numVertices; float xCoord, float yCoord; public: void set(float x, float y, int nV); }; Polygon Rectangle Triangle class Rectangle{ protected: int numVertices; float xCoord, float yCoord; public: void set(float x, float y, int nV); float area(); }; class Rectangle : public Polygon{ public: float area(); };

16. Inheritance Concept class Polygon{ protected: int numVertices; float xCoord, float yCoord; public: void set(float x, float y, int nV); }; Polygon Rectangle Triangle class Triangle{ protected: int numVertices; float xCoord, float yCoord; public: void set(float x, float y, int nV); float area(); }; class Triangle : public Polygon{ public: float area(); };

17. Inheritance Concept class Point{ protected: int x, y; public: void set (int a, int b); }; x y Point Circle 3D-Point x y r x y z class Circle : public Point{ private: double r; }; class 3D-Point: public Point{ private: int z; };

18. Inheritance Concept • Augmenting the original class • Specializing the original class Polygon Point Rectangle Triangle Circle 3D-Point real imag ComplexNumber RealNumber ImaginaryNumber imag real

19. Why Inheritance ? Inheritance is a mechanism for • building class types from existing class types • defining new class types to be a • specialization • augmentation of existing types

20. Define a Class Hierarchy • Syntax: classDerivedClassName : access-levelBaseClassName where • access-level specifies the type of derivation • private by default, or • public • Any class can serve as a base class • Thus a derived class can also be a base class

21. Class Derivation Point class Point{ protected: int x, y; public: void set (int a, int b); }; 3D-Point Sphere class 3D-Point : public Point{ private: double z; … … }; class Sphere : public 3D-Point{ private: double r; … … }; Point is the base class of 3D-Point, while 3D-Point is the base class of Sphere

22. What to inherit? • In principle, every member of a base class is inherited by a derived class • just with different access permission

23. Two levels of access control over class members class definition inheritance type Access Control Over the Members class Point{ protected: int x, y; public: void set(int a, int b); }; class Circle : public Point{ … … };

24. What to inherit? • In principle, every member of a base class is inherited by a derived class • just with different access permission • However, there are exceptions for • constructor and destructor • operator=() member • friends Since all these functions are class-specific

25. Constructor Rules for Derived Classes The default constructor and the destructor of the base class are always called when a new object of a derived class is created or destroyed. class A { public: A ( ) {cout<< “A:default”<<endl;} A (int a) {cout<<“A:parameter”<<endl;} }; class B : public A { public: B (int a) {cout<<“B”<<endl;} }; output: A:default B B test(1);

26. Constructor Rules for Derived Classes • DerivedClassCon ( derivedClass args ) : BaseClassCon ( baseClass args ) • { DerivedClass constructor body } You can also specify an constructor of the base class other than the default constructor class A { public: A ( ) {cout<< “A:default”<<endl;} A (int a) {cout<<“A:parameter”<<endl;} }; class C : public A { public: C (int a) : A(a) {cout<<“C”<<endl;} }; output: A:parameter C C test(1);

27. Define its Own Members The derived class can also define its own members, in addition to the members inherited from the base class class Point{ protected: int x, y; public: void set(int a, int b); }; x y Point x y r class Circle{ protected: int x, y; private: double r; public: void set(int a, int b); void set_r(double c); }; Circle class Circle : public Point{ private: double r; public: void set_r(double c); };

28. Even more … • A derived class can override methods defined in its parent class. With overriding, • the method in the subclass has the identical signature to the method in the base class. • a subclass implements its own version of a base class method. class A { protected: int x, y; public: void print () {cout<<“From A”<<endl;} }; class B : public A { public: void print () {cout<<“From B”<<endl;} };

29. Access a Method class Point{ protected: int x, y; public: void set(int a, int b) {x=a; y=b;} void foo (); void print(); }; class Circle : public Point{ private: double r; public: void set (int a, int b, double c) { Point :: set(a, b); //same name function call r = c; } void print(); }; Circle C; C.set(10,10,100); // from class Circle C.foo (); // from base class Point C.print(); // from class Circle Point A; A.set(30,50); // from base class Point A.print(); // from base class Point

30. Time ExtTime Putting Them Together • Time is the base class • ExtTime is the derived class with public inheritance • The derived class can • inherit all members from the base class, except for the constructors • access all public and protected members of the base class • define its private data member • provide its own constructor • define its public member functions • override functions inherited from the base class

31. classTime Specification // SPECIFICATION FILE ( time.h) class Time{ public : void Set (int h,int m, int s ) ; void Increment ( ) ; void Write ( ) const ; Time ( int initH, int initM, int initS ) ; // constructor Time () ; // default constructor protected : int hrs ; int mins ; int secs ; } ;

32. Class Interface Diagram Timeclass Set Protected data: hrs mins secs Increment Write Time Time

33. Derived Class ExtTime // SPECIFICATION FILE ( exttime.h) #include “time.h” enum ZoneType {EST, CST, MST, PST, EDT, CDT, MDT, PDT } ; class ExtTime : public Time // Time is the base class and use public inheritance { public : void Set (int h, int m, int s, ZoneType timeZone ) ; void Write ( ) const; //overridden ExtTime (int initH, int initM, int initS, ZoneType initZone ) ; ExtTime (); // default constructor private : ZoneType zone ; // added data member } ;

34. Class Interface Diagram ExtTimeclass Set Set Protected data: hrs mins secs Increment Increment Write Write ExtTime Time ExtTime Time Private data: zone

35. Implementation of ExtTime Default Constructor ExtTime :: ExtTime ( ) { zone = EST ; } ExtTime et1; et1 hrs = 0 mins = 0 secs = 0 zone = EST The default constructor of base class, Time(), is automatically called, when an ExtTime object is created.

36. 5000 hrs = 8 mins = 30 secs = 0 zone = EST et2 6000 ??? 5000 Implementation of ExtTime Another Constructor ExtTime :: ExtTime (int initH, int initM, int initS, ZoneType initZone) : Time (initH, initM, initS) // constructor initializer { zone = initZone ; } ExtTime *et2 = new ExtTime(8,30,0,EST);

37. Implementation of ExtTime void ExtTime :: Set (int h, int m, int s, ZoneType timeZone) { Time :: Set (hours, minutes, seconds); // same name function call zone = timeZone ; } void ExtTime :: Write ( ) const // function overriding { string zoneString[8] = {“EST”, “CST”, MST”, “PST”, “EDT”, “CDT”, “MDT”, “PDT”} ; Time :: Write ( ) ; cout <<‘ ‘<<zoneString[zone]<<endl; }

38. Working withExtTime #include “exttime.h” … … int main() { ExtTime thisTime ( 8, 35, 0, PST ) ; ExtTime thatTime ; // default constructor called thatTime.Write( ) ; // outputs 00:00:00 EST thatTime.Set (16, 49, 23, CDT) ; thatTime.Write( ) ; // outputs 16:49:23 CDT thisTime.Increment ( ) ; thisTime.Increment ( ) ; thisTime.Write ( ) ; // outputs 08:35:02 PST }

39. Take Home Message • Inheritance is a mechanism for defining new class types to be a specialization or an augmentation of existing types. • In principle, every member of a base class is inherited by a derived class with different access permissions, except for the constructors

40. Example: Composition class X { private: int m_Data; // data member public: X() {} // default constructor SetX (int k) // member function { m_Data = k; } }; class Y { private: int m_Data;

41. Example: Composition (contd.) public: X x; // composition Y() {} //default constructor }; void main() { Y y; //creating object of class Y y.x.SetX(20); // Access/sets the embedded object }

42. Class hierarchy from a C++ implementation of the shape example

43. Adding a subclass • Add a new subclass of Shape to allow the treelike diagram: