Access vtkActor construction object and append new data - c++

Question

I am working with VTK-6.1 and I want to create a single vtkActor using multiple vtkUnstructuredGrid. First I create the actor from a grid...
vtkSmartPointer<vtkActor> create_actor()
{
vtkSmartPointer<vtkUnstructuredGrid> grid = create_grid_a();
auto mapper = vtkSmartPointer<vtkDataSetMapper>::New();
mapper->SetInputData(grid);
auto actor = vtkSmartPointer<vtkActor>::New();
actor->SetMapper(mapper);
actor->GetProperty()->SetColor(0.5, 0.5, 0.5);
actor->GetProperty()->SetEdgeVisibility(1);
return actor;
}
and then I (try) to update actor using another grid:
void update_actor(<vtkSmartPointer<vtkActor> actor)
{
// get current grid
vtkAlgorithm* algorithm = actor->GetMapper()->GetInputAlgorithm();
actor_grid = dynamic_cast<vtkUnstructuredGrid>(algorithm);
// create new grid
vtkSmartPointer<vtkUnstructuredGrid> new_grid = create_grid_b();
// combine grids
auto append_filter = vtkSmartPointer<vtkAppendFilter>::New();
append_filter->AddInputData(actor_grid);
append_filter->AddInputData(new_grid);
append_filter->Update();
// update actor
auto mapper = vtkSmartPointer<vtkDataSetMapper>::New();
mapper->SetInputData(append_filter->GetOutput());
actor->SetMapper(mapper);
}
The problem is that the actor does not contain the combination of grids.
Notes:
(create_grid_x() functions are a little too complex to provide but they
certainly work because first actor is created correctly)
SafeDownCast as shown in 1 is not working too.
References:
vtkActor reverse access
Appending data

Answer 1

The problematic section was the retrieval of vtkUnstructuredGrid from vtkActor.
// get current grid
vtkDataSet* actor_data = actor->getMapper()->GetInput();
...instead of using vtkAlgorithm