TITLE: Polymorphic drawing devices


PROBLEM: sally@cattell.psych.upenn.edu (Sally Davis)

	I have geometric classes for lines, arcs, bezier curves,
ellipses, etc.  I also have several formats in which I want to
output various coordinates, feedrates, colors, linetype, etc.
For example, I need to output to CAD programs, CNC machines, and
plotters.  So, how would I best design a class hierarchy
that elegantly outputs the information?


RESPONSE: p j l @graceland.att.com (Paul J. Lucas), 7 Dec 94

	class AbstractRenderEngine {
	public:
		virtual void Line( int x1, int y1, int x2, int y2 ) = 0;
		virtual void Arc( int x, int y, int start, int end ) = 0;
		// ...
	};

	class CAD_RenderEngine : public AbstractRenderEngine {
	public:
		// override all pure virtuals
	};

	class CNC_RenderEngine : public AbstractRenderEngine {
	public:
		// override all pure virtuals
	};

	class Plotter_RenderEngine : public AbstractRenderEngine {
	public:
		// override all pure virtuals
	};

	class Renderer {
	public:
		Renderer( AbstractRenderEngine *e ) : engine( e ) { }
		~Renderer() { delete engine; }

		Renderer& operator=( AbstractRenderEngine *e ) {
			delete engine;
			engine = e;
			return *this;
		}

		AbstractRenderEngine* operator->() { return engine; }
	private:
		AbstractRenderEngine *engine;
	};

	/*

	This allows you to have a single Renderer class into which any
	RenderEngine cab be "plugged" and can even be changed on the
	fly at run-time.  

	*/

[ We use a design similar to the above in the graphics unit in
  guide. It encapsulates the differences between rendering to the
  screen using X, to postscript files, etc. -adc ]