Write a funtion in C++ following the above instruction. Details would be really helpful.

void reverse(int): Given a non-negative int argument say, val), this function reverses the order of the first val elements of the list, and leaves the others unchanged. Suppose our list is 10 20 30 40. The function reverse (2) would lead to 20 10 30 40, while the function reverse (3) leads to 30 20 10 40. If the argument is more than the length of the list, we reverse the entire list