default template

Let's say I have a template

template<int S = 4096>
struct A {
....
}
 
why can't be used as just A? For example, see the following program (which

doesn't compile because "A" is used instead, of A<10>, for instance):

template <int S=4096> struct A { enum {size = S}; };

int main(int argc, char* argv[]) { 
 printf("%d\n",A::size); 
}

Thanks, Kev