[vtkusers] Why can we still instantiate vtkRenderWindow though it is abstract class?

[wayne5ning]

Usually, we instantiate our window in the following way:
vtkSmartPointer<vtkRenderWindow> renderWnd  = vtkRenderWindow::New();

But when I look into the declaration of vtkRenderWindow, I find that it's an
abstract class as it has pure abstract member functions like 
virtual void Start() = 0;

So I'm confused because I've learned that abstract classes cannot be
instantiated, but here the instantiation succeeds. 

Then when I examine the class reference of its subclass 
http://www.vtk.org/doc/nightly/html/classvtkOpenGLRenderWindow.html 
vtkOpenGLRenderWindow , I find this:

vtkOpenGLRenderWindow is a concrete implementation of the abstract class
vtkRenderWindow. vtkOpenGLRenderer interfaces to the OpenGL graphics
library. Application programmers should normally use vtkRenderWindow instead
of the OpenGL specific version.

The suggestion "Application programmers should normally use vtkRenderWindow
instead of the OpenGL specific version." makes me more confused.




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