>::New();
>
> myInteractorStyle->SetImageViewer(imageViewer);
> ...
> ...
> }
>
> ```
I have tried but i am strucked by how to define `vtkStandardNewMacro()`
Can someone help me?
Thanks!
---
### Author:  [@Amnesie](https://discourse.vtk.org/u/Amnesie)
#### Post date: [September 16, 2020, 12:42am UTC](https://discourse.vtk.org/t/how-write-a-own-template-class-which-inherits-from-an-existing-vtk-class/4195/2 "2020-09-16T00:42:18Z")
5days 😭
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [September 16, 2020, 3:13am UTC](https://discourse.vtk.org/t/how-write-a-own-template-class-which-inherits-from-an-existing-vtk-class/4195/3 "2020-09-16T03:13:02Z")
My understanding is that templates are not permitted in public API of VTK classes, only in internal implementation (see [VTK coding standards](https://vtk.org/Wiki/VTK_Coding_Standards)).
This is not a limitation, but a really nice feature. Although templated classes can be easier to write and maintain for VTK _developers_, they are generally less convenient for VTK _users_ (templated implementations are more rigid, as you need to commit to specific types at compile time).
For example, look at how much simpler to use your vtkMyInteractorStyleImage class without templates in the public API:
```auto
vtkNew myInteractorStyle;
myInteractorStyle->SetViewer(someImageViewer);
```
Also see how flexible this class is - you can dynamically change between viewer classes at runtime, regardless of their type:
```auto
myInteractorStyle->SetViewer(someResliceImageViewer);
```
---
### Author:  [@Amnesie](https://discourse.vtk.org/u/Amnesie)
#### Post date: [September 16, 2020, 10:53am UTC](https://discourse.vtk.org/t/how-write-a-own-template-class-which-inherits-from-an-existing-vtk-class/4195/4 "2020-09-16T10:53:44Z")
Thanks! I didn’t know I couldn’t use template classes. From your reply, My understanding is that I can write 2 `SetViewer()` as overloading function, am I right?
But if so ,how to initialize the member of class vtkMyInteractorStyleImage?
Assume I wrote my own `InteractorStyle` class.
The function of this style is when I move forward the mouse wheel then change the slice.
The vtkMyInteractorStyleImage Class like this:
> class myVtkInteractorStyleImage : public vtkInteractorStyleImage
> {
> public:
> static myVtkInteractorStyleImage\* New();
> vtkTypeMacro(myVtkInteractorStyleImage, vtkInteractorStyleImage);
> protected:
> int \_Slice;
> int \_MinSlice;
> int \_MaxSlice;
> vtkimageviewer2 \* \_ImageViewer;
> public:
> void Setviewer(vtkimageviewer2 \* Viewer){
> \_ImageViewer = Viewer;
> \_MinSlice = imageViewer-\>GetSliceMin();
> \_MaxSlice = imageViewer-\>GetSliceMax();
> \_Slice = \_MinSlice;
> }
>
> ```
> void MoveSliceForward() {
> if(_Slice < _MaxSlice) {
> _Slice += 1;
> cout << "MoveSliceForward::Slice = " << _Slice << std::endl;
> _ImageViewer->SetSlice(_Slice);
> _ImageViewer->Render();
> }
> }
> };
> vtkStandardNewMacro(myVtkInteractorStyleImage)
>
> ```
For this class I would like to use for `vtkResliceImageViewer` as well.
How can I change the code to adapt to `vtkResliceImageViewer` class.?
---
### Author:  [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [September 16, 2020, 12:21pm UTC](https://discourse.vtk.org/t/how-write-a-own-template-class-which-inherits-from-an-existing-vtk-class/4195/5 "2020-09-16T12:21:52Z")
It’ll look something like this:
```auto
void vtkMyInteractorStyleImage::MoveSliceForward()
{
vtkResliceImageViewer* resliceImageViewer = vtkResliceImageViewer::SafeDownCast(this->ImageViewer);
vtkImageViewer2* imageViewer = vtkImageViewer2::SafeDownCast(this->ImageViewer);
if (resliceImageViewer)
{
resliceImageViewer->SetSlice(...);
...
}
else if (imageViewer)
{
imageViewer->SetSliceSomeOtherWay(...);
...
}
else
{
vtkErrorMacro("vtkMyInteractorStyleImage::MoveSliceForward failed: Invalid ImageViewer")
}
};
```
---
### Author:  [@Amnesie](https://discourse.vtk.org/u/Amnesie)
#### Post date: [September 16, 2020, 2:36pm UTC](https://discourse.vtk.org/t/how-write-a-own-template-class-which-inherits-from-an-existing-vtk-class/4195/6 "2020-09-16T14:36:18Z")
`SafeDownCast(this->ImageViewer)` is the “key” .
I should have thought of that, thank you very much!