[vtkusers] Why GetOutput is not a virtual method?

[Chen Fu]

Is there any reason make GetOutput is not a virtual
method? 
In my implement i try to use vtkImageSource* to
replace his various children class for SetInput and
GetOutput. But at last i find it impossible for
GetOutput is not a virtual method.
Why take such a design?

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Remote Scensing Satellite Ground Station
Chinese Academy of Science

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