Here was my earlier answer to this:
As Daniel indicates, you can’t access anything inside the Integrate
class unless you include its header. However, since ev_set() is not
public, you will still get an error. Currently, only classes
that derive from Integrate (like Verlet, Respa) invoke ev_set(),
so that is why it is organized that way.
You have 2 choices. Make your class a friend of Integrate,
or move ev_set() into the public section of the header.