Inheritance and Polymorphism: Two Marks Important Questions and Answers

Data Structures using C ++:

Chapter 2: Inheritance and Polymorphism

Two Marks Questions with Answers

 

1. What is an abstract class ?

Answer:

The abstract class is a base class in which at least one pure virtual function is used. Abstract classes can be used to specify an interface that must be implemented by all subclasses.

For example

class area {

double dim1,dim2;

public:

void setarea(double d1,double d2)

{

   dim=d1;

   dim=d2;

}

void getdim(double &d1,double &d2)

{

   d1=dim1;

   d2=dim2;

}

virtual double getarea()=0;//pure virtual function

};

Now we can derive another class using the area as the base class. In this derived class the pure virtual function getarea() can be redefined for calculating area of particular shape.

2. What does 'this' pointer point to do?

OR

What is role of 'this' pointer?

Answer:  

The this pointer is a hidden pointer inside every class member function that points to the class object the member function is working with. The this pointer is passed as a hidden parameter to the member function call and it is available in the member function definition as a local variable.

For example

class test

{

private:

int num;

public:

void get _val(int num)

{

   this‒>num=num;

}

}

void main()

{

   test obj;

   int num=10;

   obj.get_val(num);//here value 10 will be assigned to this‒>num

}

In above program, the obj.get_val(num) will be interpreted as get_val(&obj,num).

3. What is a virtual function ?

Answer:

A virtual function is a function that is declared within a base class and redefined by derived class. The keyword virtual must be preceeded to the virtual function.

4. When do we make a virtual function "pure" ?

OR

Define pure virtual function and mention its usage.

Answer:  

The pure virtual function is used in abstract class. When we want to override the definition of some particular function in the derived class then we normally declare that function as pure virtual function in the base class.

5. What effects do the visibility labels private, protected and public have on the members of the class ?

Answer:

If the base class has the members possessing the visibility label as private then those members are not accessible to derived class. If the members of the class are labelled as protected then that means these members are public to derived class but are private to rest of the program. If the members of the class are labelled as public then that indicates the members can be accessible by any class in the entire program.

6. What is inheritance? What are its advantages ?

Answer:

Inheritance is a process in which derived class takes some properties of the base class. At the same time derived class can have its own properties.

Advantages

1. Major advantage of inheritance is reusability which means that we can add additional features into an existing class without modifying it. For this purpose, a new class can be derived from the existing one.

2. Code extensibility is another advantage of using inheritance. The modifications can be made in the base class in order to extend the business logic.

3. The inheritance offers the Maintainability. That means if the inheritance is used then the hierarchy of base class and derived class gets created and the task of error finding becomes easy.

7. What does multiple inheritance mean?

Answer:

The multiple inheritance is a type of inheritance in which the derived class is derived from more than one class.

8. What is the default access specifier of a class?

Answer:

The default access specifier is private.

9. Specify the situation in which protected member is used.

Answer:

When the members of a class should be accessible by its immediate derived class (child class) and not by the derived class of derived class(grand child class) then in that situation the protected members are used.

10. What is multilevel inheritance ?

Answer:

When the derived class is derived from a base class which itself is derived class then that type of inheritance is called multilevel inheritance.

11. How can a private member be made inheritable?

Answer:

We can not inherit the private members of a class, but we can use friend function to access the private members.

12. What is the difference between private, protected and public modes ?

Answer:

If the base class have private members then those members are not accessible to derived class. Protected members are public to derived classes but private to reset of the program. And the public members are accessible to all.

13. What is the difference between compile time and run time polymorphism?

Answer:

Compile time polymorphism : The linking between method call and method definition takes place at compile time. Compiler knows the exact memory address of the function definition. The advantage of this approach is that the execution of the code becomes fast.

For example ‒ Function oveloading

Run time polymorphism : The linking between method call and method definition takes place at run time. The advantage of this approach is that size of compiled code gets less.

For example ‒ Virtual functions.

14. What is function overriding ?

Answer:

The function overriding means redefining of a function in a derived class .

15. What is the need to declare base class as virtual ?

Answer:

When a derived class is derived from two classes and these two classes are themselves derived from one base class, then ambiguity may occur while using the member functions of parent classes because two copies of the parent classes are included in the derived class. To avoid this problem, the base class is declared as virtual.

16. What is the use of virtual function in C++ ?

Answer:

Virtual function allows to have one interface, multiple functions performing different tasks. Thus virtual function allows to have polymorphic feature of object oriented programming.

17. Is it possible to inherit the constructor of Its base class? Justify.

Answer:

No, it is not possible to inherit the constructor of its base class.

18. What is the purpose of composition ?

Answer:

Combining or assembling the existing components together, instead of creating a new class is called composition.

19. List out various forms of inheritance.

Answer:

1. Single inheritance

2. Multilevel inheritance

3. Multiple inheritance

4. Hybrid inheritance

20. Write a C++ code to display as area of square or rectangle using function overriding.

Answer:

#include<iostream>

using namespace std;

class Rectangle

{

   private:

   double length,bredth;

   public:

   void setlengthBredth( double l,double b);

   void computeArea();

};

void Rectangle::setlengthBredth(double 1,double b)

{

   length=1;

   bredth=b;

}

void Rectangle::computeArea()

{

   double area;

   area=length *bredth;

   cout<<"\n Area of Rectangle = "<<area;

}

class Square:public Rectangle

{

   private:

   double length;

   public:

   void setlength( double 1);

   void computeArea();

};

void Square::setlength(double 1)

{

   length=1;

}

void Square::computeArea()

{

   double area;

   area = length*length;

   cout<<"\n Area of Square = "<<area;

}

int main()

{

   Rectangle obj1;

   Square obj2;

   obj1.setlengthBredth(10,20);

   obj1.computeArea();

   obj2.setlength(10);

   obj2.computeArea();//function overriding

   return 0;

}

Output

Area of Rectangle = 200

Area of Square = 100

21. Write a sample code to show the usage of this pointer in C++.

Answer:

#include<iostream>

#include<cstring>

using namespace std;

class Test

{

   private:

   char fname[30];

   public:

   void getData(char *str)

   {

      strcpy(fname, str);

   }

   void display(void)

   {

      cout<<"Name:" << this‒>fname << endl;

   }

   void displaywithout_this(void)

   {

      cout << "Name:"<< fname << endl;

   }

};

void main(void)

{

   Test obj;

   obj.getData("Sachin");

   obj.display();

   obj.displaywithout_this();

}

Output

Name:Sachin

Name:Sachin

22. Differentiate between private and protected members of a class.

Answer:

• If the base class have private members then those members are not accessible to derived class.

• On the other hand, protected members are public to derived class but private to rest of the program.

23. What is dynamic binding ?

Answer:

• C++ provides the facility to specify that the compiler should match function calls with the correct definition at the run time.

• This mechanism is called dynamic binding. Dynamic binding is a mechanism due to which the compiler has a provision to resolve the call to appropriate function late until the run time.

• Dynamic binding is achieved using virtual function.

24. Write a C++ program to demonstrate the virtual function.

Answer:

#include <iostream>

using namespace std;

class Base

{

   public:

   virtual void display()

   {

      cout << "\n In Base Class":

   }

};

class Derived:public Base

{

    public:

   void display()

   {

      cout << "\n In Derived Class";

   }

};

void main()

{

   Base *ptr;

   Derived d;

   ptr = &d;

   ptr‒>display();

}

Output

In Derived Class

25. State about Cast Operator.

Answer:

Cast operator is used to convert one data type element into another.

Various cast operators are –

1) const_cast<type><expression>

2) dynamic_cast t<type><expression>

3) static_cast<type>(expression)

4) reinterprete_cast<type>(expression)

26. Give the syntax of a pointer to a function which returns an integer and takes arguments one of integer type and 2 of float type. What is the difference between a class and a structure ?

Answer:

Syntax

int (*ptr)(int, float, float);

Difference between class and structure:

Class

1. It is mainly used for object oriented programming.

2. In class, we have data, functions, constructors,destructors and inheritance.

3. More secure because we can declare data as private

Structure

1. It is mainly used for grouping data.

2. Traditionally we store only data in structure.

3. Less secure as all the members are public.

27. When is protected access specifier used in a class?

Answer:

Protected members are used when data or functions should be hidden from outside code but still available to subclasses for inheritance.

28. State the purpose of virtual base class.

Answer:

Virtual base classes are used to avoid multiple copies of the same base class when using multiple inheritance.