August 1997
OP216: OBJECT ORIENTED PROGRAMMING

QUESTION 3

Total Marks: 20 Marks

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3. (a) Polymorphism is an important characteristic of object-oriented programming; it can be defined in C++ using virtual functions. Define a virtual function example that takes an integer parameter and returns void. [3]
virtual void ExampleFunction(int);
deduct one mark for each syntax error or omission, minimum zero.
[3 marks]
(b) The use of virtual functions allows dynamic binding. Explain what is meant by dynamic binding and explain how it is different from static binding. [4]
Dynamic binding is the association of a name to a specific function/item at runtime. [2]
Static binding is undertaken at compile time, whereas dynamic binding is done at runtime. [2]
[4 marks]
(c) State one advantage and one disadvantage of dynamic binding. [2]
Advantages:
Dynamic binding allows more freedom when an algorithm is specified.
It permits the system to automatically determine the right function to call, based on the type of the object, at runtime.
one mark for either of these
Disadvantage: Performance penalty is incurred as it involves an extra lookup at runtime. [1]
[2 marks]
(d) What is a friend function? [2]
A friend function is a function declared to have access to the protected and private members of a class. [2]
(e) In the context of friend functions, give a reasoned answer to the following questions.
(i) if Dog class is a friend of Boy class, is Boy a friend of Dog? [3]
No [1]
friendship is not commutative. [2]
(ii) if Dog is a friend of Boy, and Terrier derives from Dog, is Terrier a friend of Boy? [3]
No [1]
friendship is not inherited. [2]
(iii) if Dog is a friend of Boy, and Boy is a friend of House, is Dog a friend of House? [3]
No [1]
friendship is not associative. [2]
in each case, one mark for a correct answer, and two marks for a suitable explanation
[9 marks]