### Author:  [@nicoco](https://discourse.vtk.org/u/nicoco)
#### Post date: [July 15, 2019, 3:28pm UTC](https://discourse.vtk.org/t/invalid-use-of-incomplete-type-class-vtkpolydata/1337/1 "2019-07-15T15:28:22Z")
Hello everyone,
I would like to test whether an array is present in a polydata, but I am getting this error at compile time:
```
NicocoTest.cxx:10:39: error: invalid use of incomplete type ‘class vtkPolyData’
if (reader.GetPointer()->GetOutput()->GetPointData()->HasArray("ARRAYNAME")) {
^~
In file included from NicocoTest.cxx:1: VTK-8.2.0/IO/Legacy/vtkPolyDataReader.h:37:7: note: forward declaration of ‘class vtkPolyData’
class vtkPolyData;
^ ~~~~~~~~~~
```
Here is the source code leading to this hard-to-understand-for-c++-noob error:
```
#include "vtkPolyDataReader.h"
#include "vtkNew.h"
int main(int argc, char *argv[]) {
std::cout << "Reading file" << std::endl;
vtkNew reader;
reader->SetFileName(argv[1]);
if (reader.GetPointer()->GetOutput()->GetPointData()->HasArray("ARRAYNAME")) {
std:cout << "An array named ARRAYNAME was found" << std::endl;
}
return EXIT_SUCCESS;
}
```
What am I doing wrong? What would be the correct way of avoid this error?
Thank you very much.
---
### Author:  [@danlipsa](https://discourse.vtk.org/u/danlipsa)
#### Post date: [July 15, 2019, 4:00pm UTC](https://discourse.vtk.org/t/invalid-use-of-incomplete-type-class-vtkpolydata/1337/2 "2019-07-15T16:00:22Z")
I think you’ll have to
#include “vtkPolyData.h”
---
### Author:  [@lplavorante](https://discourse.vtk.org/u/lplavorante)
#### Post date: [July 15, 2019, 4:13pm UTC](https://discourse.vtk.org/t/invalid-use-of-incomplete-type-class-vtkpolydata/1337/3 "2019-07-15T16:13:22Z")
Nicoco, after
reader-\>SetFileName(argv[1]);
insert the line
reader-\>Update();
This method is necessary to actually perform the reading and update correctly reader-\>GetOutput().
One question: what are you going to do with reader-\>GetOutput() ?
Maybe, you could do something like (this will copy your data):
vtkNew\< vtkPolyData \> polyData;
polyData-\>DeepCopy(reader-\>GetOutput());
Or simply
vtkSmartPointer\< vtkPolyData \> polyData = reader-\>GetOutput(); // No copy here
And then use the object ’ polyData’ for the rest of your processing / visualization pipeline.
Hope this help.
```
Luca
```
---
### Author:  [@nicoco](https://discourse.vtk.org/u/nicoco)
#### Post date: [July 16, 2019, 6:08am UTC](https://discourse.vtk.org/t/invalid-use-of-incomplete-type-class-vtkpolydata/1337/4 "2019-07-16T06:08:02Z")
Thank you for your help. Unfortunately, this does not solve the compile-time issue at all. I first tried just adding `include`. Here is what the code looks like now:
```
#include "vtkPolyDataReader.h"
#include "vtkNew.h"
#include "vtkPolyData.h"
int main(int argc, char *argv[]) {
std::cout << "Reading file" << std::endl;
vtkNew reader;
reader->SetFileName(argv[1]);
reader->Update();
vtkSmartPointer< vtkPolyData > polyData = reader->GetOutput();
int hasArray = polyData->GetPointData()->HasArray("ARRAYNAME");
if (hasArray) {
std:cout << "An array named ARRAYNAME was found" << std::endl;
}
return EXIT_SUCCESS;
}
```
and the error:
```
NicocoTest.cxx: In function ‘int main(int, char**)’:
NicocoTest.cxx:14:42: error: invalid use of incomplete type ‘class vtkPointData’
int hasArray = polyData->GetPointData()->HasArray("ARRAYNAME");
^~
In file included from VTK-8.2.0/Common/DataModel/vtkPointSet.h:33,
from Common/DataModel/vtkPolyData.h:62,
from NicocoTest.cxx:3:
VTK-8.2.0/Common/DataModel/vtkDataSet.h:52:7: note: forward declaration of ‘class vtkPointData’
class vtkPointData;
^ ~~~~~~~~~~~
make[2]: *** [nicoco/CMakeFiles/NicocoTest.dir/build.make:63: nicoco/CMakeFiles/NicocoTest.dir/NicocoTest.cxx.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:1233: nicoco/CMakeFiles/NicocoTest.dir/all] Error 2
make: *** [Makefile:95: all] Error 2
```
What I am trying to achieve seems so basic…
---
### Author:  [@ben.boeckel](https://discourse.vtk.org/u/ben.boeckel)
#### Post date: [July 16, 2019, 8:39am UTC](https://discourse.vtk.org/t/invalid-use-of-incomplete-type-class-vtkpolydata/1337/5 "2019-07-16T08:39:25Z")
> [@nicoco](#):
>
> error: invalid use of incomplete type ‘class vtkPointData’
This error pretty much always means that header inclusions are missing. In VTK, the header name is (in the vast majority of cases) `classname.h`, so here, adding `#include "vtkPointData.h"` would make the class declaration available.
---
### Author:  [@nicoco](https://discourse.vtk.org/u/nicoco)
#### Post date: [July 16, 2019, 8:57am UTC](https://discourse.vtk.org/t/invalid-use-of-incomplete-type-class-vtkpolydata/1337/6 "2019-07-16T08:57:04Z")
Thank you very much @ben.boeckel
That was it!