C++

Lecture 7Thursday, 21 July 2005

Chapter 9 Inheritance

Creating new classes from the existing classes

The notions of base classes and derived classes

Basic Idea of Inheritance

A base object A has some properties (data and functions).

An derived object B from A can have the same set of data or functions, plus some of its own.

Additionally, some of the data or functions may not be available for B, or some functions redefined.

Class Hierarchy

Shape

2D-shape 3D-shape

Circle

Quadrilateral

Triangle Sphere

Cube

Square

Inheritance, example

A Point class has two coordinates (x,y), there are functions to get and set the x and y values.

A Circle class derived from Point is a Point plus the data radius, and additional functions of getting area, setting and getting radius.

Three Type of Members

public: can be accessed outside member function, including derived class

private: off limit except for member functions

protected: can not accessed outside member functions, except by derived class

Point/Circle Example

class Point { … protected: int x, y;};class Circle : public Point { …}

Class that Contains other Classes

class Employee { public: … private: Date birthday; Date hireday;}

Classes that is Derived from Existing Classes

class Point { ….}

class Circle : public Point { …}

Three Types of Class Members

public - accessible by member and non-member functions

private - accessible by class member functions and its friends

protected - accessible by member function, its friends, and derived class, but not non-member functions

Point Base Class

class Point { public: Point(int=0, int=0); void setPoint(int, int); int getX(); int getY(); protected: int x, y;};

Point Class

The Point class can be used asPoint A, B(1,0);A.setPoint(2,4);x = A.getX();

but NOTxvalue = A.x; // because x is protected

Circle Class Derived from Point Class

class Circle : public Point { friend ostream &operator<< …public: Circle(double r=0.0, int x=0, int y=0); void setRadius(double); double getRadius() const; double area() const;protected: double radius;}

Circle Class

We can use the circle class in the following ways

Circle c;c.setPoint(1,2);x = c.getX();y = c.getY();c.getRadius(1.2);a = c.area();

c.f. Fig. 9.4-6 (points);

9.7-9.9 (circle);

9.12-9.16 (point2,circle3)

Case Study, Point, Circle, Cylinder

A Point class contains x, y A Circle is a point with radius r A Cylinder is a Circle plus a height

C.f. Fig. 9.22-24

Member Type and Inheritance Type

class A : public (or protected or private) B {

public: - - - protected: - - - private:}

Inheritance Type

Base class member type

Public inheritance

Protected inheritance

Private inheritance

public No change Protected in derived class

Private in derived class

protected No change Protected in derived class

Private in derived class

private Hidden in derived class

Hidden in derived class

Hidden in derived class

See also Fig. 9.30

Multiple Inheritance

base1 base2

Derived-Class

Chapter 10

Virtual Functions and Polymorphism

Basic Idea

Let the classes Square, Circle, etc be derived from the base class Shape.

We want to be able to draw all the shapes with

p->draw(); where p is a pointer of type Shape

class.

Draw a lot of Shapes

We might want to draw many different shapes by a loopfor(j = 0; j < jmax; ++j) {

p[j] -> draw();}

where p is array of pointers of base type Shape each pointing to a different class of concrete shape.

Virtual Functions

Declare a virtual function in the base classvirtual void draw() const;

Declare as “virtual” again in derived class if over-riding is needed.

Define the function draw() for each of the derived classes.

Dynamical binding

Pure Virtual Function

Declarevirtual function prototype = 0;

A pure virtual function is declared in the base class but not implemented. Implementations are given in specific derived class.

Class Inheritance and (Virtual) Functions

Shape

Point

Circle

Cylinder

(Pure) Virtual functions declared in Shape

Virtual functions maybe implemented in Point, Circle, or Cylinder

Base class

Derived classes

The Shape Example

shape.h (Fig. 10.13)

point.h, point.cpp (Fig.10.14-15)

circle.h, circle.cpp (Fig.10.16-17)

cylindr.h, cylindr.cpp (Fig.10.18-19)

Main program (Fig.10.20)

C.f. Fig. 10.13-20

Questions

What are virtual functions?

Distinguish between static and dynamic binding.

Distinguish between virtual functions and pure virtual functions.