December 1998 OP216: Object Oriented Programming (Question 2)

(a)	List three types of polymorphism found in C++.	[3]
	(1 mark) for any three of: coercion, inclusion, overloading, parametric.
(b)	Explain the benefits of polymorphism in object-oriented programming.	[2]
	Adding a new type does not require new code (1 mark). One name can be used to refer to one concept (1 mark). Award (1 mark) for any alternative benefit.
(c)	Consider the following declaration of a Stack in C++: template <class T> class Stack { private: ... \\ Some declarations public: Stack(); \\ Constructor that initialises the stack void push(T) \\ place an item at the top of the stack T pop(); \\ remove and return the top item on the stack int size(); \\ return the number of items in the stack int empty(); \\ return 1 if the stack is empty, otherwise return 0 }
	(i) The polymorphic function peek takes a reference to a non-empty stack s and returns the top item of s without discarding any items in s. Copy and complete the following definition of peek. template <class T> T peek(Stack<T> & s) { };	[4]
	template <class T> T peek(Stack<T> & s) { T item; [1] item = s.pop(); [1] s.push(item); [1] return item; [1] }; For alternative solutions, the following scheme should be used: local variable declaration of type T (1 mark); assignment of top item to item (1 mark); pushing the top item back on the stack (1 mark); returning the item (1 mark).
	(ii) The polymorphic function reverse takes a reference to a stack s and changes it so that its items are in reverse order. Copy and complete the following definition of reverse. template <class T> void reverse(Stack<T> & s) { };	[6]
	template <class T> void reverse(Stack<T> & s) { Stack<T> t; [1] while (!s.empty()) [2] t.push (!s.pop()); [2] s = t; [1] }; Local variable declaration of type Stack<T> (1 mark); use of loop (1 mark) with appropriate guard (1 mark); pop item from s (1 mark) and push it on t (1 mark); assign the local variable to s (1 mark).
	(iii) The polymorphic function drop takes a reference to a stack s and an integer n, and removes the first n items from s. If n is greater than the number of items in s, then all items in s are removed. Copy and complete the following definition of drop. template <class T> void drop(Stack<T> & s, int n) { };	[5]
	template <class T> void drop(Stack<T> & s, int n) { if (n>s.size()) [1] n = s.size(); [1] for (; n != 0; n--) [2] s.pop(); [1] }; Test the size of the stack (1 mark); limit n to size of stack (1 mark); each correct loop guard and decrement (1 mark); use of pop in loop (1 mark).