Hello,
It is not advised to implement the constructor by hand for classes derived from VTK classes. Instead, use a VTK macro to correct use a VTK object factory:
// Implementation of the New() function for this class.
vtkStandardNewMacro(CustomInteractorStyle);
Place this as a free standing line of code in a .cpp file (not inside class declaration nor definition).
If you still want to declare and define the class in a single shot, please, find how to correctly implement the New() function here: Header-only vtk-derived class .
best,
PC