Code fails with linker error. Why?

[John Colvin via Digitalmars-d-learn]

On Friday, 3 October 2014 at 15:44:16 UTC, eles wrote:
> class ShapeSurface(T) {
> public:
> 	int formula();

that means you have a definition of formula elsewhere (which the 
linker tries to find, but obviously fails. What you want is

class ShapeSurface(T) {
public:
	abstract int formula();