Quote Originally Posted by jerome12345 View Post
Error 5 error LNK2019: unresolved external symbol ffunc referenced in function "public: int __cdecl calculator::apply_cb(void)" (?apply_cb@calculator@@QEAAHXZ) calculator.obj calculator
This error message doesn't reference your Fortran function at all, unless I'm mistaken. So I suspect the actual problem is unrelated to the Fortran aspect, at least for the most part.