I found a solution.
I create the class in Class.cpp.
Then I create two header.
first one Class.hpp I define the object here.
then second one ClassExtern.hpp I define the sam object extern.
And I include the Class.cpp both of them.
After that I include the Class.hpp in main.cpp and ClassExtern.hpp in function.cpp.
Then it work.
But I think there has to be easier way.