C++Talk.NET

Amadeus W. M. · Guest

Sometimes - more often than I would like - I run into situations
where I need a member function to be simultaneously templated and
virtual. I know it can be done, but I'm wondering if there's
some clever design trick that can circumvent this problem.

Example:

template <class Pixel_t>
class Image;

class BaseShape
{
public:

template <class Pixel_t>
void draw(Image<Pixel_t> & I); // draw self onto image.

};

class Circle : public BaseShape
{
public:

template <class Pixel_t>
void draw(Image<Pixel_t> & I); // circle specific.
};

etc.

I have a

list<BaseShape*> Shapes;

which I draw in a sweep:

list<BaseShape*>::iterator s;
for(s=Shapes.begin(); s!=Shapes.end(); s++)
s->draw(I);

This requires that draw() be virtual. Note that nothing else
but draw() needs to know anything about the pixel type (the
template parameter of Image), so I don't want to make the entire
BaseImage a template. Also, since Image is templated, I can't
derive all Images from one NON-templated common, abstract
BaseImage (and define a virtual void draw(BaseImage & I) const =0).

Is there any way to do what I want? I suspect there may be some
design flaw, otherwise I wouldn't be getting to this problem,
but I really like the idea of having each shape draw itself on
the image. If only the image was of a single type.