Run-time type information and vtkWhatever::New()

[Brian Alexander Todd]

Dear vtkusers,

I have a question about navigating the vtk class heirarchy.
I have an application that uses a "document/view" architecture.
The doc maintains a list of <vtkDataSet *> and the views render
the data using pipelines that depend on what the exact type of
vtkDataSet is (vtkStructuredPoints, vtkPolyData, etc.).  At somepoint,
the view is required to figure out exactly what kind of object the
vtkDataSet pointer points to so it can decide which pipeline to use.  
Naturally, I assumed I should use dynamic_cast to do this, since the 
type will not be known at compile-time.  I ran into some problems doing
this

If I run this block of code under WinNT 4.0 (SP 5) and VC++ 6.0 (SP 3),

	vtkDataSet * vtkds;
	vtkStructuredPoints * vtksp1 = vtkStructuredPoints::New();
	vtkStructuredPoints * vtksp2;

	vtkds = vtksp1;
	vtksp2 = dynamic_cast<vtkStructuredPoints *>( vtkds );

	std::cout << vtksp1 << std::endl;
	std::cout << vtksp2 << std::endl;

The program returns,

	00300570
	00000000  

This confused me because it means the program was unable to 
convert back to the subclass vtkStructuredPoints after handing
the pointer off to the polymorphic vtkDataSet.  However, if
I through out the vtkStructuredPoints::New idea and go with the
normal "new vtkStructuredPoints", as in,

	vtkDataSet * vtkds;
	vtkStructuredPoints * vtksp1 = new vtkStructuredPoints;
	vtkStructuredPoints * vtksp2;

	vtkds = vtksp1;
	vtksp2 = dynamic_cast<vtkStructuredPoints *>( vtkds );

	std::cout << vtksp1 << std::endl;
	std::cout << vtksp2 << std::endl;

then the program returns,

	00300570
	00300570

which is the expected behavior.

So, basically, I have two questions.  Why does this happen (out of
curiousoty) and what can I do to get run-time type identification
so that make my program works (more importantly)?

Thank you,
Brian Todd
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