Inheritance, Composition CSCI 103L

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Inheritance, Composition

CSCI 103L

Jeffrey Miller, Ph.D.

LECTURE 16

HTTP://WWW-SCF.USC.EDU/~CSCI103B

USC CSCI 103L

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Outline

USC CSCI 103L 2/26

▪ Inheritance

▪ Instantiating Objects with Inheritance

▪ Inheritance Example

▪ Program

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Inheritance

▪ Inheritance is a form of software reuse in which you create a class that absorbs an existing class’s data and behaviors and enhances them with new

capabilities

› Functions and variables from a parent class that are not private will be inherited into the child class

▪ When creating a class, you can designate that the new class should inherit the members of an existing class

› The existing class is called the base class (or parent class)

› The new class is called the derived class (or child class)

USC CSCI 103L 3/26 Inheritance

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Punnett Square

▪ In biology, we determine inheritance using a Punnett Square, looking at genotypes and alleles

› If one parent has the genotype BB and the other bb, what is the likelihood the child will have bb?

› If one parent has the genotype Bb and the other bb, what is the likelihood the child will have bb?

B B

b Bb Bb

B b

b Bb bb

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Single vs Multiple Inheritance

▪ Single inheritance allows a derived class to inherit from only one base class

› Java supports single inheritance

▪ Multiple inheritance allows a derived class to inherit from more than one base class

› C++ supports multiple inheritance

› There is a potential problem with multiple inheritance if more than one base class provides an implementation for the same function

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Inheritance vs Composition

▪ is-a Relationship

› If an object has an “is-a” relationship with another object, inheritance will be used

› Vehicle, Car, Truck, Motorcycle

▪ has-a Relationship

› If an object has a “has-a” relationship with another object, composition will be used

› Car, Steering Wheel, Brake Pedal, Speedometer

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Inheritance Hierarchy

▪ To show a child class and a parent class in a

diagram, we draw a line connecting the child class to the parent class where the parent class is above the child

Shape

2-D Shape 3-D Shape

Triangle Rectangle

Circle

Square Sphere

Cone Box

Cube

Ellipse Ellipsoid

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Class Inheritance Types

▪ When inheriting from a class, you specify the type of inheritance you would like in addition to the name of the class from which to inherit

▪ The type of inheritance (public, protected, or private) will determine how you inherit variables and

functions from the parent class

class TwoDShape : public Shape

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Class Inheritance Types (cont.)

▪ Public Inheritance

› With public inheritance, all of the base class members retain their original member access when they become members of the derived class

› This means private members are still private to the base class, so the derived class can NOT access them

▪ Protected Inheritance

› With protected inheritance, public and protected members of the base class become protected members of the derived class

› Private members are still private to the base class

▪ Private Inheritance

› With private inheritance, public and protected members of the base class become private members of the derived class

› Private members are still private to the base class

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Outline

USC CSCI 103L 10/26

▪ Inheritance

▪ Instantiating Objects with Inheritance

▪ Inheritance Example

▪ Program

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Parent and Child Instantiation

▪ When a child class is instantiated, the parent class must be instantiated first in the child class’s constructor

› This will happen automatically by the compiler calling the parent class’s default constructor unless we explicitly instantiate the parent

› Note that if there is no default constructor in the parent, we MUST explicitly call the parent class’s constructor from the child

▪ When we call the parent class’s constructor from the child, it must be placed immediately after the parameter list of the constructor in the child

USC CSCI 103L 11/26 Instantiating Objects with Inheritance

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Default Constructor in Parent #1

1 #include <iostream>

2 using namespace std;

3

4 class Parent { 5 public:

6 Parent() {

7 cout << "in parent" << endl;

8 } 9 };

10

11 class Child : public Parent { 12 public:

13 Child() {

14 cout << "in child" << endl;

15 } 16 };

17

18 int main() { 19 Child c;

20 return 1;

21 }

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Default Constructor in Parent #2

3

4 class Parent { 5 public:

6 Parent() {

8 } 9 };

10

13 Child(int num) {

15 } 16 };

17

18 int main() { 19 Child c(3);

20 return 1;

21 }

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Non-Default Constructor in Parent #1

3

4 class Parent { 5 private:

6 int num;

7 public:

8 Parent(int num) {

10 this->num = num;

11 } 12 };

13

16 Child() {

18 } 19 };

20

21 int main() { 22 Child c;

23 return 1;

24 }

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Non-Default Constructor in Parent #2

3

4 class Parent { 5 private:

6 int num;

7 public:

8 Parent(int num) {

10 this->num = num;

11 } 12 };

13

16 Child(int n) : Parent(n) { 17 cout << "in child" << endl;

18 } 19 };

20

21 int main() { 22 Child c(3);

23 return 1;

24 }

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Non-Default Constructor in Parent #3

3

4 class Parent { 5 private:

6 int num;

7 public:

8 Parent(int num) {

10 this->num = num;

11 } 12 };

13

16 Child() : Parent(3) {

18 } 19 };

20

21 int main() { 22 Child c;

23 return 1;

24 }

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Outline

USC CSCI 103L 17/26

▪ Inheritance

▪ Instantiating Objects with Inheritance

▪ Inheritance Example

▪ Program

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Single Class Example

3

4 class Shape { 5 protected:

6 string name;

7 public:

8 Shape(string name) { 9 this->name = name;

10 }

11 void print_name() { 12 cout << this->name;

13 } 14 };

15

16 int main() {

17 Shape s("csci 103");

18 s.print_name();

19 return 1;

20 }

USC CSCI 103L 18/26 Inheritance Example

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Two Class Example #1

3

6 string name;

7 public:

8 Shape() { }

11 }

14 } 15 };

16

17 class TwoDShape : public Shape { 18 public:

19 TwoDShape(string name) { 20 this->name = name;

21 } 22 };

23

24 int main() {

25 TwoDShape tds("csci 103");

26 tds.print_name();

27 return 1;

28 }

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Two Class Example #2

3

6 string name;

7 public:

8 Shape() { }

11 }

14 } 15 };

16

17 class TwoDShape : public Shape { 18 public:

19 TwoDShape(string name) { 20 Shape::name = name;

21 } 22 };

23

24 int main() {

25 TwoDShape tds("csci 103");

26 tds.print_name();

27 return 1;

28 }

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Three Class Example

3

6 string name;

7 public:

8 Shape() { }

11 }

14 } 15 };

16

19 TwoDShape(string name) { 20 this->name = name;

21 } 22 };

23 class Triangle : public TwoDShape { 24 private:

25 double base, height;

26 public:

27 Triangle(string n, double b, double h) : TwoDShape(n) { 28 this->base = b;

29 this->height = h;

30 }

31 float get_area() {

32 return 0.5 * base * height;

33 } 34 };

35

36 int main() {

37 Triangle tri("triangle", 10.0, 5.0);

38 tri.print_name();

39 cout << " has area = " << tri.get_area() << endl;

40 return 1;

41 }

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Four Class Example

3

6 string name;

7 public:

8 Shape() { }

11 }

14 } 15 };

16

21 } 22 };

23 class Triangle : public TwoDShape { 24 private:

25 double base, height;

26 public:

27 Triangle(string n, double b, double h) : TwoDShape(n) { 28 this->base = b;

29 this->height = h;

30 }

31 double get_area() {

32 return 0.5 * base * height;

33 } 34 };

35

36 class Rectangle : public TwoDShape { 37 private:

38 double height, width;

39 public:

40 Rectangle(string n, double h, double w) : TwoDShape(n) { 41 this->height = h;

42 this->width = w;

43 }

44 double get_area() { 45 return height * width;

46 } 47 };

48

49 int main() {

50 Triangle tri("triangle", 10.0, 5.0);

51 tri.print_name();

52 cout << "area = " << tri.get_area() << endl;

53 Rectangle rect("rectangle", 10.0, 5.0);

54 rect.print_name();

55 cout << " has area = " << rect.get_area() << endl;

56 return 1;

57 }

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Five Class Example

3

6 string name;

7 public:

8 Shape() { }

11 }

14 } 15 };

16

21 } 22 };

26 public:

30 }

33 } 34 };

35

36 class Rectangle : public TwoDShape { 37 private:

38 double height, width;

39 public:

40 Rectangle(string n, double h, double w) : TwoDShape(n) { 41 this->height = h;

42 this->width = w;

43 }

44 double get_area() { 45 return height * width;

46 } 47 };

48

49 class Square : public Rectangle { 50 public:

51 Square(string n, double s) : Rectangle(n, s, s) { } 52 };

53

54 int main() {

56 tri.print_name();

57 cout << " has area = " << tri.get_area() << endl;

58 Rectangle rect("rectangle", 10.0, 5.0);

59 rect.print_name();

60 cout << " has area = " << rect.get_area() << endl;

61 Square sq("square", 10.0);

62 sq.print_name();

63 cout << " area = " << sq.get_area() << endl;

64 return 1;

65 }

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Passing Inherited Objects

3

6 string name;

7 public:

8 Shape() { }

11 }

14 } 15 };

16

21 } 22 };

26 public:

30 }

33 } 34 };

35

36 class Rectangle : public TwoDShape { 37 private:

38 double height, width;

39 public:

40 Rectangle(string n, double h, double w) : TwoDShape(n) { 41 this->height = h;

42 this->width = w;

43 }

44 double get_area() { 45 return height * width;

46 } 47 };

48

49 class Square : public Rectangle { 50 public:

51 Square(string n, double s) : Rectangle(n, s, s) { } 52 };

53

54 void print_shape_name(Shape s) { 55 cout << "The shape ";

56 s.print_name();

57 } 58

59 int main() {

61 print_shape_name(tri);

62 cout << " has area = " << tri.get_area() << endl;

63 Rectangle rect("rectangle", 10.0, 5.0);

64 print_shape_name(rect);

65 cout << " has area = " << rect.get_area() << endl;

66 Square sq("square", 10.0);

67 print_shape_name(sq);

68 cout << " has area = " << sq.get_area() << endl;

69 return 1;

70 }

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Outline

USC CSCI 103L 25/26

▪ Inheritance

▪ Instantiating Objects with Inheritance

▪ Inheritance Example

▪ Program

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Program

▪ Mimicking the program in this lecture, create a program for finding the volume of three-dimensional shapes. Make sure to draw out the inheritance hierarchy you will use for cube, box, sphere, threeDShape, and Shape before writing any code.

USC CSCI 103L 26/26 Program

c:\>ThreeDShapes.exe

Enter the radius of the sphere: 3 Volume of sphere is 113.10

Enter the side of the cube: 5 Volume of cube is 125

Enter the width of the box: 3 Enter the length of the box: 4 Enter the height of the box: 5 Volume of box is 60

c:\>