It’s not possible to perform type-based logic (w/ templates), because the compiler does not consider branches and must make sure that a template class or function performs no invalid operations for the type it is instantiated with (use overloading, template specializations, or traits).

template<typename Itr, typename Distance> void advance(Itr itr, const Distance& n) {     if (itr is a random_access_iterator)     {         // Compile-time error, itr += not valid         // for forward iterator, even though at         // run-time this check would not be performed         itr += n;     }         else     {         while(n--) itr++;     } } int main(int argc, char * argv []) {     std::list l{1,2,3};         std::list::iterator itr = l.begin();         advance(itr, 2); // Error