15 - Polymorphism and Virtual Functions

Question 1

What is polymorphism?

Answer 1

Polymorphism refers to the ability to associate many meanings to one function name by means of a special mechanism known as virtual functions or late binding.

Question 2

When you make a function virtual, you are telling the compiler:

Answer 2

“I do not know how this function is implemented. Wait until it is used in a program, and then get the implementation from the object instance.”

Question 3

The technique of waiting until run time to determine the implementation of a procedure is often called ..

Answer 3

late binding or dynamic binding.

Question 4

Which programming languages have virtual functions as a default?

Answer 4

Java.

Question 5

Why are virtual functions not default with C++?

Answer 5

Doing so uses more storage and makes your program run slower.

Question 6

Undefined reference to Class_Name virtual table.

That’s an error message. How was it produced?

Answer 6

If any virtual member functions are not implemented before compiling, the compilation fails with those types of error messages.

Question 7

Suppose you are designing software for a graphics package that has classes for several figures, such as rectangles, circles, ovals, and so forth. Show how virtual functions are useful.

Answer 7

Each figure might be an object of a different class, such as the Rectangle class or the Circle class. In a well-designed programming project, all of these classes would probably be descendants of a single parent class called, for example, Figure. Now, suppose you want a function to draw a figure on the screen. To draw a circle, you need different instructions from those you need to draw a rectangle. So, each class needs to have a different function to draw its kind of figure. If r is a Rectangle object and c is a Circle object, then r.draw( ) and c.draw( ) can be functions implemented with different code. The parent class Figure may have a function called center that moves a figure to the center of the screen by erasing it and then redrawing it in the center of the screen. The function Figure::center might use the function draw to redraw the figure to the center of the screen. By making the member function draw a virtual function, you can write code for the member function Figure::center in the class Figure and know that when it is used for a derived class - say, Circle - the definition of draw in the class Circle will be the definition used.

Question 8

Suppose you are designing software for a graphics package that has classes for several figures, such as rectangles, circles, ovals, and so forth. You want a virtual function draw ( ) in a parent class Figure. How do you implement it?

Answer 8

You’re never going to need to draw a Figure that is not using the virtual function of the subclass, and so you need to make the Figure draw function a pure virtual function.
The way to make a member function into a pure virtual function is to mark it as virtual and to add the annotation = 0 to the member function declaration.

virtual void draw ( ) = 0;

Question 9

A class has this code somewhere in its header file. 
virtual void draw ( ) = 0;
What would you call this class?

Answer 9

This is a pure virtual function. A class with one or more pure virtual functions is called an abstract class.