How to write last recursion for meta-programming template in c++

Question

I wrote following metaprogramming template:

template <unsigned int N, unsigned int P>
struct cutom_imagined
{ 
    static unsigned int function(unsigned int r)
    {
        return (P + N + r) * cutom_imagined<N - 1>::function(r);
    }
};

P is actually like a constant. How should I write last recursion for above example? I suppose it should look similar to this one:

template <>
struct cutom_imagined<0, /* What should be here? */ >
{ 
    static unsigned int function(unsigned int) { return 1; }
};

But don't know how to write it...

Answer 1

Make P part of the template and the specialization. First off, the recursive call is:

return (P + N + r) * cutom_imagined<N - 1, P>::function(r);

Second, the specialziation is now partial:

template <unsigned int P>
struct cutom_imagined<0, P>
{
    static unsigned int function(unsigned int) { return 1; }
};

Answer 2

If your compiler supports constexpr you can also do it this like:

constexpr unsigned int func(unsigned int r, unsigned int p, unsigned int n)
{
  return (n == 0) ? 1 : ((p + n + r) * func(r, p, n - 1));
}