# How to determine the CMake VTK "COMPONENTS" necessary for a class?
**URL:** https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429
**Category:** Development
**Created:** [May 10, 2023, 4:29pm UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429 "2023-05-10T16:29:24Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [May 10, 2023, 4:29pm UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/1 "2023-05-10T16:29:24Z")
I haven’t written a CMakeLists.txt file for VTK in a while, and things seem to have evolved somewhat. I’m sure that the reasons are important, and the results are great, but it did take me a while to figure out why I was getting link errors on `vtkXMLPolyDataReader`. My original code:
```auto
find_package(VTK COMPONENTS
CommonCore)
```
The fixed code:
```auto
find_package(VTK COMPONENTS
CommonCore
IOXML)
```
Granted `IOXML` is fairly easy to figure out, but there is nothing on the [documentation page](https://vtk.org/doc/nightly/html/classvtkXMLPolyDataReader.html) that mentions that I needed this …
---
### Author:  [@Christos\_Tsolakis](https://discourse.vtk.org/u/Christos_Tsolakis)
#### Post date: [May 10, 2023, 5:10pm UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/2 "2023-05-10T17:10:15Z")
Hi @rexthor ,
since you are using `vtkXMLPolyDataReader` you need to include the vtk module that defines this class. Notice that in the VTK source tree the header is located under `IO/XML` which is where the `IOXML` name comes from.
Another way to find the module name is the [vtk.module](https://gitlab.kitware.com/vtk/vtk/-/blob/master/IO/XML/vtk.module) file in the same directory with the header. This is where the module is defined.
Check also this [script](https://kitware.github.io/vtk-examples/site/Python/Utilities/VTKModulesForCxx) from vtk-examples website that creates the “find\_package” string for you.
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### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [May 11, 2023, 7:22am UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/3 "2023-05-11T07:22:01Z")
Hi agree with @rexthor , the needed module dependencies should be listed in the class doc.
Should be added in the automatic documentation process imo.
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### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [May 11, 2023, 12:08pm UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/4 "2023-05-11T12:08:32Z")
Since people in general are quick to say what they don’t like, and often don’t say what they do like: VTK has _great_ documentation, and even lots of examples … so … kudos to all involved.
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### Author:  [@ben.boeckel](https://discourse.vtk.org/u/ben.boeckel)
#### Post date: [May 11, 2023, 12:37pm UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/5 "2023-05-11T12:37:00Z")
> [@mwestphal](#):
>
> Should be added in the automatic documentation process imo.
Note that some classes will require module X to compile, but to get real implementations, another module will be needed (e.g., `VTK::RenderingOpenGL2` is rarely used directly rather than via `VTK::RenderingCore` factory classes).
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### Author:  [@mwestphal](https://discourse.vtk.org/u/mwestphal)
#### Post date: [May 11, 2023, 12:46pm UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/6 "2023-05-11T12:46:30Z")
Good point, but better to have something than nothing I think.
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### Author:  [@amaclean](https://discourse.vtk.org/u/amaclean)
#### Post date: [May 12, 2023, 5:47am UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/7 "2023-05-12T05:47:40Z")
This is why [VTKModulesForCxx](https://kitware.github.io/vtk-examples/site/Python/Utilities/VTKModulesForCxx/) generates a series of commented out modules that the user may need to uncomment.
e.g.
```plaintext
# These modules are suggested since they implement an existing module.
# You may need to uncomment one or more of these.
# If vtkRenderWindow is used and you want to use OpenGL,
# you also need the RenderingOpenGL2 module.
# If vtkRenderWindowInteractor is used,
# uncomment RenderingUI and possibly InteractionStyle.
# If text rendering is used, uncomment RenderingFreeType
#
# FiltersParallel # implements VTK::FiltersCore
# FiltersParallelDIY2 # implements VTK::FiltersCore
# RenderingCellGrid # implements VTK::RenderingCore
# RenderingFreeType # implements VTK::RenderingCore
# RenderingOpenGL2 # implements VTK::RenderingCore
# RenderingUI # implements VTK::RenderingCore
```
SImilarly with [VTKImportsForPython](https://kitware.github.io/vtk-examples/site/Python/Utilities/VTKImportsForPython/) you get:
```plaintext
# You may need to uncomment one or more of the following imports.
# If vtkRenderWindow is used and you want to use OpenGL,
# you also need the vtkRenderingOpenGL2 module.
# If vtkRenderWindowInteractor is used, uncomment vtkInteractionStyle
# If text rendering is used, uncomment vtkRenderingFreeType.
#
# If using PyCharm, preface each one you select with this line:
# noinspection PyUnresolvedReferences
#
# import vtkmodules.vtkInteractionStyle
# import vtkmodules.vtkRenderingContextOpenGL2
# import vtkmodules.vtkRenderingFreeType
# import vtkmodules.vtkRenderingOpenGL2
# import vtkmodules.vtkRenderingUI
# import vtkmodules.vtkRenderingVolumeOpenGL2
```
I’m not sure if this can be improved on.
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### Author:  [@rexthor](https://discourse.vtk.org/u/rexthor)
#### Post date: [May 12, 2023, 1:39pm UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/8 "2023-05-12T13:39:47Z")
As infrequently as I make new C++ projects, the script solution you guys mention would work for me. I can only imagine how much of a headache it would be to go back and annotate all of the documents.
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### Author:  [@fghoussen](https://discourse.vtk.org/u/fghoussen)
#### Post date: [May 24, 2023, 5:54pm UTC](https://discourse.vtk.org/t/how-to-determine-the-cmake-vtk-components-necessary-for-a-class/11429/9 "2023-05-24T17:54:45Z")
> [@Christos\_Tsolakis](#):
>
> ples website that creates
`python3 "$VTK_ROOT/Utilities/Maintenance/FindNeededModules.py" -j $VTK_ROOT/build/modules.json -s vtkHelloworld.cpp` worked for me: [VTK-9: How to know which VTK COMPONENT contains which VTK class?](https://discourse.vtk.org/t/vtk-9-how-to-know-which-vtk-component-contains-which-vtk-class/11533).
Not sure what is the recommended way to go