This fails, if I create object in include file:

int main(int argc, char *argv[])
{
...

#   include "createFields.H"
...

interface.initializeControlPointsPosition(); 
...
--------------------------------------------------


This however works:
int main(int argc, char *argv[])
{
...
...
...

freeSurface interface(mesh, rho, U, p, phi);
interface.initializeControlPointsPosition();
...
-------------------------------------------------
Here, if I type "interface." and press Strg-Tab , code completion offers functions, also "initializeControlPointsPosition"
No include errors appear.

"freeSurface interface(mesh, rho, U, p, phi);" 
is part of 
#   include "createFields.H"