1. Chapter 10 Inheritance and Polymorphism
C/C++ Language Programming
Wanxiang Che

2. Array of Objects // Initialize sum double sum = 0; // Add areas to sum
Circle circles[2] = {Circle(3), Circle(4)};
// Initialize sum
double sum = 0;
// Add areas to sum
for (int i = 0; i < 2; i++)
sum+=circles[i].get_area();

3. How to sum different Shape?
Circle circle(3);
Rectangle rect(4, 5);
double sum = 0;
sum += circle.get_area();
sum += rect.get_area();

4. How to sum different Shape?
Shape shapes[2] = {Circle(3), Rectangle(4, 5)};
// Initialize sum
double sum = 0;
// Add areas to sum
for (int i = 0; i < 2; i++)
sum += shapes[i].get_area();

5. Inheritance Concept
Derive a new class (subclass) from an existing class (base class or superclass).
Inheritance creates a hierarchy of related classes (types) which share code and interface.

6. Shape class hierarchy Shape inherits (isa) Rectangle • • • • Circle
Square
Triangle
Circle
• • • •
inherits (isa)

7. Inheritance UML

8. Define a Class Hierarchy
Syntax:
class DerivedClassName : access-level BaseClassName
access-level specifies the type of derivation
private by default, or public
E.g. class Circle: public Shape
Any class can serve as a base class
Thus a derived class can also be a base class

9. Redefining Functions
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;} }

10. redefining vs. overloading
Overloading a function
A way to provide more than one function with the same name but with different signatures to distinguish them.
Redefining a function
The function must be defined in the derived class using the same signature and same return type as in its base class

11. Static Binding Classes X print() inherits (isa) Y print() Z print()
X x; Y y; Z z; X *px; px = & ??; // can be x,y,or z px->print(); // ??
print()
inherits (isa) Y
print() Z
print()

12. Two Types of Binding Static Binding (the default in C++)
px->print() uses X’s print
this is known at compile time
Dynamic Binding
px->print() uses the print() in the object pointed at
this is only known at run time
with virtual functions

13. Why “only known at run time”?
Assume dynamic binding is being used: X x; Y y; Z z; X *px; cin >> val; if (val == 1) px = &x; else px = &y; px->print(); // which print() is used?

14. Define Virtual Functions
To enable dynamic binding for a function, need to do two things:
The function must be declared virtual in the base class.
The variable that references the object for the function must contain the address of the object.

15. Virtual Function class Shape { public: Shape();
Shape(const Shape& orig);
virtual ~Shape();
void print();
virtual double get_area(); };

16. Solution
int main() {
Shape *shapes[2] = {new Circle(3), new Rectangle(4, 5)};
double sum = 0;
for(int i = 0; i < 2; i++){
sum += shapes[i]->get_area(); }
cout << sum << endl;
return 0;

17. Note
If a function is defined virtual in a base class, it is automatically virtual in all its derived classes. It is not necessary to add the keyword virtual in the function declaration in the derived class.