// example of virtual method			  file:   virtual.cc
// a Drawing is an ordered list of Shapes
// There are various Shapes - all subclasses of Shape
// To render() a Drawing you draw() each Shape* in the list
// Compiler can't tell at compile time which draw() method to use
// The SRGP (Simplified rastor Graphics System) is used to draw.
// This is a C library based on X11 which requires an X server
//   g++ virtual.cc -lsrgp -lX11

extern "C" {
#include "/users/faculty/sedgwick/C/srgp.h"  //Graphics library
}

class Shape {protected:  int x, y; //all shapes have one Point
 public:   //protected accessible by subclasses
   Shape(int x1, int y1):x(x1), y(y1) {}                            //ctor
   void virtual draw()=0; //pure virtual - all subclasses must implement
};

class Rect: public Shape {protected:  int x2, y2;
 public:
   Rect(int x1, int y1, int x2, int y2):Shape(x1,y1),x2(x2),y2(y2){}//ctor
   void draw() { SRGP_rectangleCoord (x,y, x2,y2); }
};

class Oval: public Rect {
 public:
   Oval(int x1, int y1, int x2, int y2) : Rect(x1,y1, x2,y2) {}     //ctor
   void draw(){ SRGP_ellipse( SRGP_defRectangle(x,y,x2,y2) ); }
};

class Drawing { 
   Shape *p;       // could point to Oval or Rect
   Drawing * next; // remaining elements of the Drawing
 public:
   Drawing(Shape *q, Drawing *r){p = q; next = r;}                  //ctor
   void render()
    { p->draw();                  // draw first element 
      if ( next ) next->render(); // render rest if any
    }
};

int main()
{ Oval o1(100,100,200,200), o2(300,300,400,400);
  Rect r (50,50, 450,450);

  SRGP_begin("Drawing Window",500,500,1,FALSE); // open drawing window

  Drawing d1(&r, 0),   //   d       d2      d1
	  d2(&o2, &d1),// [o1|,-]-->[o2|,-]-->[r| ,0]  linked list of shapes
          d(&o1, &d2); 

  d.render();  // will draw o1 then o2 and finally r

  /* At end of program wait for user to hit RETURN key before closing window*/
  SRGP_setInputMode (KEYBOARD, EVENT); /* start monitoring keybd events */

  SRGP_waitEvent(-1); /* wait indefinitely for RETURN key */
} /* Graphics Window disappears here */