April 1999
OP216: OBJECT-ORIENTED PROGRAMMING

QUESTION 4

Total Marks: 20 Marks

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GRADE A
Sample student's solutions are indicated in green.
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(a) Explain what is meant by inheritance in object-oriented programming and give one benefit of it. [3]
Inheritance
It is the derivation of a new data type based on an existing data type. For instance, a derived class can be created according to a base class.

Benefit
It saves the re-entering of code through reusability.

 
Singapore Post requires a program to simulate its postal system. You are to write such a program.
(b) An item of mail contains a postage cost (an integer representing cents) and the type of the mail (a pointer to an array of characters). Give an implementation of the class Mail that contains: (1) these private data items; (2) an accessor for the postage cost; and (3) a method display that prints the mail and the postage cost in the form $x.xx, e.g. the cost 100 would be displayed as $1.00. [5]
class Mail
{   private:
       int postage_cost;
       char *type_of_mail;
    public:
       int access_cost()
       { return postage_cost;}
       void display()
       { cout << "Type of mail: " <<   
        type_of_mail << endl;
        cout << "Cost: $" << postage_cost;
       }
};

 
(c) A package is an item of mail that has postage $3.50 and a integer weight. Use inheritance to implement the class Package that contains a constructor that takes a weight and initialises all its variables appropriately. [6]
class Package: public Mail
{   private:
       int weights;
    public:
       Package(int a): Package.weight(a),
       Package.postage_cost,
       Package.type_of_mail,
       {  weight = a;
          type_of_mail = "Package";
          postage_cost = 350;
       }
};

 
(d) A mailbox is a collection of mail items. Consider the following declaration of the class Mailbox.

class MailBox {
private:
...                // Some private declarations
public:
   MailBox();       // The constructor
   Mail current(); // Returns the current mail item
                    // in the collection
   void next();     // Move to the next item in the
                    // collection
    int empty();    // Return 1 if all the mail items
                    // have been looked at, or 0
                    // otherwise.

Give the implementation of a function total_postage that takes a reference to a mailbox and returns the total postage of all mail items in it.

 [6]
Assumption: Th collection of mail items is stored in an array of type Mail.
Declaration:
Mail array[100];

int MailBox::total_postage(MailBox &a)
{   int x = 0;
    int i;
    for (i=0;i<100;i++)
       x += array[i].postage_cost;
    return x;
};