Data Structures - A Templated Stack Data Structure Example

       #include < dos.h > // For sleep()
  #include < iostream.h > // For I/0
  #include < windows.h > // FOR MessageBox() API
  #include < conio.h >
  #define MAX 10 // MAXIMUM STACK CONTENT
  template < class T > // Using Templates so that any type of data can be // stored in Stack without multiple defination of class
  class stack
  {
   protected:
     T arr[MAX]; // Contains all the Data
   public:
     T item,r;
     int top; //Contains location of Topmost Data pushed onto Stack
     stack() //Constructor
     {
       for(int i=0;i< MAX;i++)
       {
         arr[i]=NULL; //Initialises all Stack Contents to NULL
       }
       top=-1; //Sets the Top Location to -1 indicating an empty stack
     }
     void push(T a) // Push ie. Add Value Function
     {
       top++; // increment to by 1
       if(top< MAX)
       {
         arr[top]=a; //If Stack is Vacant store Value in Array
       }
       else // Bug the User
       {
         MessageBox(0,"STACK IS FULL","STACK WARNING!",MB_ICONSTOP);
         top--;
       }
     }
     T pop() // Delete Item. Returns the deleted item
     {
       if(top==-1)
       {
         MessageBox(0,"STACK IS EMPTY","WARNING",MB_ICONSTOP);
         return NULL;
       }
       else
       {
         T data=arr[top]; //Set Topmost Value in data
         arr[top]=NULL; //Set Original Location to NULL
         top--; // Decrement top by 1
         return data; // Return deleted item
       }
     }
  };
  void main()
  {
     stack < int >a; // Create object of class a with int Template
     int opt=1;
     while (opt!=3)
     {
       clrscr();
       cout<<" MAX STACK CAPACITY="<<((MAX-a.top)-1)<<"";
       cout<<"1) Push Item";
       cout<<"2) Pop Item";
       cout<<"3) Exit";
       cout<<"Option?";
       cin>>opt;
       switch(opt)
       {
         case 1:
           cout<<"Which Number should be pushed?";
           cin>>a.item;
           a.push(a.item);
           break;
         case 2:
           a.r=a.pop();
           cout<<"Item popped from Stack is:"<< a.r << endl;
           sleep(2);
           break;
       }
     }
   }