A completely useless but fun-to-write example for C++ template metaprogramming. Did you ever wanted to calculate the control digit of a UIC wagon number at compile-time? This serves just as an example that any computer algorithm can be implemented in some way in a template metaprogram (template metaprogramming is generally Turing-complete).

#include <iostream>
template <long long u, int m>
struct S
{
static long long const val = S<u/10, m%2+1>::val + ((u%10)*m)/10 + ((u%10)*m)%10;
};
template <int m>
struct S<0, m>
{
static long long const val = 0;
};
template <long long u>
struct UIC
{
static long long const val = u*10+(10-S<u,2>::val%10)%10;
};
int main()
{
std::cout << UIC<21812471217LLU>::val << std::endl;
return 0;
}