[vtkusers] question about vtkAlgorithm::Update()

[dizou]

vtkAlgorithm::Update() executes the following code:

this->GetExecutive()->Update();

So I looked at vtkExecutive::Update(). This is the code for
vtkExecutive::Update():

  if (this->Algorithm->GetNumberOfOutputPorts())
    {
    return this->Update(0);
    }
  return this->Update(-1);

So vtkExecutive::Update(int) is:

int vtkExecutive::Update(int)
{
  vtkErrorMacro("This class does not implement Update.");
  return 0;
}

So does this mean that vtkAlgorithm::Update() will always have a return
value of 0? So what exactly does vtkAlgorithm::Update() end up doing?

this->Algorithm->GetNumberOfOutputPorts() returns 1

This is the code I have:

Geo = vtkGeometryFilter()
Geo.SetInput(XReader.GetOutputDataObject(0))
Normals = vtkPolyDataNormals()
Normals.SetInput(Geo.GetOutput())
Normals.Update()

I get this error message when I get to Normals.Update():
The instruction at "***" referenced memory at "***". The memory could not be
"read"

XReader.GetOutputDataObject(0) and Geo.GetOutput() all produce valid output.
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