1. Inheritance & Polymorphism
Visit for more Learning Resources

2. OOP Concepts Objects and Classes / Data Encapsulation Inheritance
Polymorphism
Operator overloading is a kind of polymorphism

3. Polymorphism
A function/operator can behave differently depending on the context in which it is called
We have already seen operator overloading in which a single ‘+’ operator behaves differently when used with floats, ints or strings
Polymorphism refers to the ability of similar objects to respond differently to the same message i.e., the same type of function call
Selecting the correct function is deferred until runtime and is based on the object

4. Upcasting Inheritance represents an “is-a” relationship
The new class is a type of the existing class
enum note { middleC, Csharp, Cflat };
class Instrument {
public:
void play(note) const {} };
// Wind objects are also Instruments
class Wind : public Instrument {};
void tune(Instrument& i) {
// ...
i.play(middleC); }
int main() {
Wind flute;
tune(flute); // Upcasting

5. Upcasting (contd..)
A Wind object is also an Instrument object, and there’s no function that tune( ) could call for an Instrument that isn’t also in Wind
Inside tune( ), the code works for Instrument and anything derived from Instrument, and the act of converting a Wind object, reference, or pointer into an Instrument object, reference, or pointer is called upcasting

6. Pointer and Reference Upcasting
Wind w;
Instrument* ip = &w; // Upcast
Instrument& ir = w; // Upcast
Of course, any upcast loses type information about an object. If you say
Instrument* ip = &w;
the compiler can deal with ip only as an Instrument pointer and nothing else i.e., it cannot know that ip actually points to a Wind object
So when you call the play( ) member function by saying
ip->play(middleC);
the compiler can know only that it’s calling play( ) for an Instrument pointer, and call the base-class version of Instrument::play( ) instead of what it should do, which is call Wind::play( )

7. Problem with upcasting
enum note { middleC, Csharp, Cflat }; class Instrument { public: void play(note) const { cout << "Instrument::play" << endl; } }; class Wind : public Instrument { cout << "Wind::play" << endl; void tune(Instrument& i) { // ... i.play(middleC); int main() { Wind flute; tune(flute); // Upcasting }

8. Problem with upcasting
The output of the previous program is
Instrument::play
This is clearly not the desired output, because you know that the object is actually a Wind and not just an Instrument
The call should resolve to Wind::play
The above discussion also applies if you use a pointer to an Instrument in the function tune

9. Function call binding
Connecting a function call to a function body is called binding
When binding is performed before the program is run (by the compiler and linker), it’s called early binding or static binding
The problem in the above program is caused by early binding because the compiler cannot know the correct function to call when it has only an Instrument address

10. Dynamic Binding
The solution to the above problem is called late binding or dynamic binding which means the binding occurs at runtime, based on the type of the object
When a language implements late binding, there must be some mechanism to determine the type of the object at runtime and call the appropriate member function
The compiler still doesn’t know the actual object type, but it inserts code that finds out and calls the correct function body
Implemented in C++ using virtual functions

11. Virtual Functions
Virtual Functions enable run time object determination
Keyword virtual instructs the compiler to use late binding and delay the object interpretation
How ?
Define a virtual function in the base class. The word virtual appears only in the base class
If a base class declares a virtual function, it must implement that function, even if the body is empty
Virtual function in base class stays virtual in all the derived classes
It can be overridden in the derived classes
But, a derived class is not required to re-implement a virtual function. If it does not, the base class version is used

12. Virtual Functions
enum note { middleC, Csharp, Cflat }; class Instrument { public: virtual void play(note) const { cout << "Instrument::play" << endl; } }; class Wind : public Instrument { void play(note) const { cout << "Wind::play" << endl; void tune(Instrument& i) { // ... i.play(middleC); int main() { Wind flute; tune(flute); // prints Wind::play

13. Extensibility
With play( ) defined as virtual in the base class, you can add as many new types as you want to the system without changing the tune( ) function
Such a program is extensible because you can add new functionality by inheriting new data types from the common base class
The functions that manipulate the base-class interface will not need to be changed at all to accommodate the new classes

14. Extensibility
enum note { middleC, Csharp, Cflat }; class Instrument { public: virtual void play(note) const { cout << "Instrument::play" << endl; } virtual char* what() const { return "Instrument"; virtual void adjust(int) {} }; class Wind : public Instrument { void play(note) const { cout << "Wind::play" << endl; char* what() const { return "Wind"; } void adjust(int) {}

15. class Percussion : public Instrument { public: void play(note) const { cout << "Percussion::play" << endl; } char* what() const { return "Percussion"; } void adjust(int) {} }; class Stringed : public Instrument { cout << "Stringed::play" << endl; char* what() const { return "Stringed"; } class Brass : public Wind { cout << "Brass::play" << endl; char* what() const { return "Brass"; } };

16. class Woodwind : public Wind { public: void play(note) const { cout << "Woodwind::play" << endl; } char* what() const { return "Woodwind"; } }; void tune(Instrument& i) { // ... i.play(middleC); int main() { Wind flute; Percussion drum; Stringed violin; Brass flugelhorn; Woodwind recorder; tune(flute); tune(drum); tune(violin); tune(flugelhorn); tune(recorder);

17. Virtual functions work only with addresses (references / pointers)
class Base {
public:
virtual int f() const { return 1; }
void g()const { cout<<“Base”; }
}; 
class Derived : public Base {
int f() const { return 2; }
void show(){ cout<<“derived only”; }
void g() { cout<<“derived”; } };

18. Virtual functions work only with addresses (references / pointers)
void main() { Derived d; Base* b1 = &d; Base& b2 = d; Base b3; // Late binding: cout << "b1->f() = " << b1->f() << endl; cout << "b2.f() = " << b2.f() << endl; // Early binding: cout << "b3.f() = " << b3.f() << endl; cout<<“b1->g() = ” ; b1->g(); cout<<endl; cout<<“b1->show()=”; b1->show(); }

19. Arrays of Pointers to Objects
class Base { public: virtual void show() { cout<<"base“; } }; class Derv1 : public Base { void show() { cout<<"derv1“; }; class Derv2 : public Base { cout<<"derv2“; }
void main() {
Base* array[2];
Derv1 d1;
Derv2 d2;
array[0] = &d1;
array[1] = &d2;
for(int i=0;i<2;i++)
array[i]->show();
getch(); }

20. Object Slicing
If you use an object instead of a pointer or reference as the recipient of your upcast, the object is “sliced”
class Base { int i; public: Base(int ii = 0) : i(ii) {} virtual int sum() const { return i; } }; class Derived : public Base { int j; Derived(int ii = 0, int jj = 0): Base(ii),j(jj) {} int sum() const { return Base::sum() + j; }};
void call(Base b) {
cout << "sum = " << b.sum() << endl; }
int main() {
Base b(10);
Derived d(10, 47);
call(b);
call(d);

21. Polymorphism Summary
When you use virtual functions, compiler stores additional information about the types of object available and created
Polymorphism is supported at this additional overhead
Important :
virtual functions work only with pointers/references
Not with objects even if the function is virtual

22. Abstract Classes & Pure Virtual Functions
Some classes exist logically but not physically.
Example : Shape
Shape s; // Legal but silly..!! : “Shapeless shape”
Shape makes sense only as a base of some classes derived from it. Serves as a “category”
Hence instantiation of such a class must be prevented
A class with one or more pure virtual functions is an Abstract Class
Objects of abstract class can’t be created
class Shape //Abstract
{ public : //Pure virtual Function virtual void draw() = 0; }
Shape s; // error : variable of an abstract class

23. Example Shape virtual void draw() Circle Triangle public void draw()

24. Abstract Classes & Pure Virtual Functions
A pure virtual function not defined in the derived class remains a pure virtual function.
Hence derived class also becomes abstract
class Circle : public Shape { //No draw() - Abstract public : void print(){ cout << “I am a circle” << endl; }
class Rectangle : public Shape { public : void draw(){ // Override Shape::draw() cout << “Drawing Rectangle” << endl; }
Rectangle r; // Valid
Circle c; // error : variable of an abstract class

25. Abstract classes
If a method is to be over-ridden by each child class, we can enforce this policy through the use of abstract classes
You can’t create an object of an abstract class ( a class with at least one pure virtual function)
You can also not pass or return an object of an abstract class by value

26. Abstract classes
It is still possible to provide definition of a pure virtual function in the base class
The class still remains abstract and functions must be redefined in the derived classes, but a common piece of code can be kept there to facilitate reuse
class Shape { //Abstract public : virtual void draw() = 0; };
Shape::draw(){ cout << “Shape" << endl; }
class Rectangle : public Shape
{ public : void draw(){ Shape::draw(); //Reuse cout <<“Rectangle”<< endl; }

27. Virtual Destructor
If any of the functions in your class are virtual, the destructor should be virtual as well
class Base {
public:
virtual ~Base() {
cout << "~Base()" << endl; } };
class Derived : public Base {
~Derived() {
cout << "~Derived()" << endl;
int main() {
Base* bp = new Derived; // Upcast
delete bp; // Virtual destructor call

28. Things to remember
Beginning a class method declaration with the keyword virtual in a base class makes the function virtual for the base class and all derived classes, classes further derived from derived classes, and so on…
dynamic binding is possible only for a base class pointer or reference to an object of the derived class
All those functions of the base class that need to be over-ridden in the derived classes should be made virtual
Constructors cannot be virtual
Destructors can be virtual
You should provide a base class which has virtual functions with a virtual destructor even if the class does not need a destructor (i.e., it does not contain dynamic data)

29. Things to remember
friends can’t be virtual since they are not member functions
If a derived class does not redefine a virtual function, the base class version is used. If there is a chain of derived classes, then the most recently defined version of the virtual function is used
An abstract class demands that its pure virtual functions must be over-ridden in each derived class, otherwise the derived class also becomes abstract

30. Inheritance and Dynamic Memory Allocation
If a base class uses dynamic memory allocation and redefines assignment operator and copy constructor, how does it affect the implementation of the derived class?
If the derived class does not itself use dynamic memory allocation, no special steps need to be taken
If the derived class uses dynamic memory allocation, then the assignment operator and copy constructor need to be defined for the derived class as well

31. If the derived class does not use new
class base { private: char* label; int rating; public: base(const char* l = "null",int r = 0); base(const base& rs); virtual ~base(); base& operator = (const base& rs); }; class lacksDMA : public base char color[40]; ...

32. If the derived class does not use new
There’s no need for defining a destructor. The default destructor will automatically call the base class destructor which will free the memory allocated in the base part of the object
There’s no need for defining a copy constructor since the default copy constructor will automatically call the base class copy constructor for copying the base part of the object
Assignment operator overloading is also not required for the same reason

33. If the derived class does use new
class hasDMA : public base {
private:
char* style; //to point to dynamic memory
public:
... };
For this class, you will have to define destructor, copy constructor and assignment operator

34. If the derived class uses new
Destructor
base::~base() {
delete [] label; }
hasDMA::~hasDMA()
delete[] style;

35. If the derived class uses new
Copy Constructor
base::base(const base& rs) {
label = new char[strlen(rs.label)+1];
strcpy(label,rs.label);
rating = rs.rating; }
hasDMA::hasDMA(const hasDMA& hs) : base(hs)
style = new char[strlen(hs.style)+1];
strcpy(style,hs.style);

36. If the derived class uses new
Assignment Operator
base& base::operator=(const base& rs) {
if(this==&rs)
return *this;
delete [] label;
label = new char[strlen(rs.label)+1];
strcpy(label,rs.label);
rating = rs.rating; }
hasDMA& hasDMA::operator=(const hasDMA& hs)
if(this==&hs)
base::operator=(hs); //copy base
style = new char[strlen(hs.style)+1];
strcpy(style,hs.style);

37. stream operators
ostream& operator<<(ostream& os,const base& rs) //declared friend in class base { os<<"Label: " <<rs.label << endl; os << "Rating: " << rs.rating << endl; return os; } ostream& operator<<(ostream& os,const lacksDMA& rs) //declared friend in lacksDMA os << (const base& )rs; //type casting //lacksDMA parameter to base argument os << "Color: " <<rs.color << endl;

38. stream operators
ostream& operator<< (ostream& os, const hasDMA& rs) //declared friend in hasDMA {
os << (const base& )rs; //type casting //hasDMA parameter to base argument
os << "Style: " <<rs.style << endl;
return os; }

39. Recommended Reading
Bruce Eckel’s “Thinking in C++” available online at

40. How C++ implements virtual binding

41. How C++ implements virtual binding

42. Cost of Polymorphism VTABLE for every polymorphic class
VPTR embedded in every polymorphic object
Indirect function access

43. Another Example class Base { public: virtual void function1() {};  
class D1: public Base
    void function1() {};
class D2: public Base
    void function2() {};

44. For more detail
contact us