# Proplem using vtkPLYWriter with WriteToOutputString = true

**URL:** https://discourse.vtk.org/t/proplem-using-vtkplywriter-with-writetooutputstring-true/10114
**Category:** Development
**Tags:** code
**Created:** [December 8, 2022, 6:31am UTC](https://discourse.vtk.org/t/proplem-using-vtkplywriter-with-writetooutputstring-true/10114 "2022-12-08T06:31:00Z")
**Posts on this page:** 4
**Page:** 1

<div class="post-metadata">

### Author: ![dwventer](https://discourse.vtk.org/user_avatar/discourse.vtk.org/dwventer/32/1579_2.png) [@dwventer](https://discourse.vtk.org/u/dwventer)
#### Post date: [December 8, 2022, 6:31am UTC](https://discourse.vtk.org/t/proplem-using-vtkplywriter-with-writetooutputstring-true/10114/1 "2022-12-08T06:31:00Z")

</div>

Hi,

I am trying to use the vtkPLYWriter to write to a std::string. This is an extraction of my code:

```
vtkNew<vtkPLYWriter> vtkWriter;
vtkWriter->WriteToOutputStringOn();
vtkWriter->SetFileTypeToBinary();
vtkWriter->SetInputData(polyDataModel);
int ret = vtkWriter->Write();
std::string dataString = vtkWriter->GetOutputString();

```

The `dataString` is always empty after execution.

I went looking in the VTK code and it looks like the vtkPLYWriter member `OutputString` does not get assigned properly.

This is the code found in VTK 9.1 (vtkPLY.cxx):

```
PlyFile* vtkPLY::ply_open_for_writing_to_string(
  std::string& output, int nelems, const char** elem_names, int file_type)
{
  PlyFile* plyfile;
  // memory leaks
  plyInitialize();
  std::ostringstream* oss = new std::ostringstream;
  oss->str(output);

  /* create the actual PlyFile structure */
  plyfile = vtkPLY::ply_write(oss, nelems, elem_names, file_type);
  if (plyfile == nullptr)
  {
    delete oss;
    return (nullptr);
  }

  /* return pointer to the file descriptor */
  return (plyfile);
}

```

As you can see it looks like `oss` gets initialised with `OutputString` but that’s it. By adding a very simple hack to the `vtkPLYWriter::WriteData` method it seemed to fix the problem.

My modification in file vtkPLYWriter.cxx:

```
void vtkPLYWriter::WriteData()
{
...
...
...
  //Added this to fix write to string
  if (this->WriteToOutputString)
  {
    std::ostringstream* ofs = dynamic_cast< std::ostringstream* >(ply->os);
    if (ofs)
      this->OutputString = ofs->str();
  }
  // close the PLY file
  vtkPLY::ply_close(ply);
}

```

My code is probably not ideal as I am no expert in the VTK code. Should someone add something similar to fix the problem?

Cheers  
Daniel

---

<div class="post-metadata">

### Author: ![lassoan](https://discourse.vtk.org/user_avatar/discourse.vtk.org/lassoan/32/50_2.png) [@lassoan](https://discourse.vtk.org/u/lassoan)
#### Post date: [December 10, 2022, 4:00am UTC](https://discourse.vtk.org/t/proplem-using-vtkplywriter-with-writetooutputstring-true/10114/2 "2022-12-10T04:00:49Z")

</div>

Thank you, it seems you have found a bug and a solution, too. To make it easier for VTK developers to review and integrate your proposed change, I would recommend to submit a merge request. See details here:

[https://vtk.org/contributing-code/](https://vtk.org/contributing-code/)

---

<div class="post-metadata">

### Author: ![toddy](https://discourse.vtk.org/letter_avatar_proxy/v4/letter/t/edb3f5/32.png) [@toddy](https://discourse.vtk.org/u/toddy)
#### Post date: [December 10, 2022, 6:00am UTC](https://discourse.vtk.org/t/proplem-using-vtkplywriter-with-writetooutputstring-true/10114/3 "2022-12-10T06:00:15Z")

</div>

I think the problem here is that  
`vtkPLY::ply_open_for_writing_to_string(std::string& output)`  
should be  
`vtkPLY::ply_open_for_writing_to_stream(std::ostream& output)`

```auto
  std::ostringstream* stringstream = nullptr;
  if (this->WriteToOutputString)
  {
    stringstream = new std::ostringstream;
    ply = vtkPLY::ply_open_for_writing_to_stream(&stringstream, ...)
  }

```

Then there is no need for the dynamic cast later just

```auto
  if (this->WriteToOutputString)
  {
    this->OutputString = stringstream->str();
    delete stringstream;
  }

```

---

<div class="post-metadata">

### Author: ![dwventer](https://discourse.vtk.org/user_avatar/discourse.vtk.org/dwventer/32/1579_2.png) [@dwventer](https://discourse.vtk.org/u/dwventer)
#### Post date: [December 11, 2022, 9:40pm UTC](https://discourse.vtk.org/t/proplem-using-vtkplywriter-with-writetooutputstring-true/10114/4 "2022-12-11T21:40:43Z")

</div>

I agree with Todd’s recommendation. Seems unnecessary to pass the `string output` to `ply_open_for_writing_to_string` if it is not going to be used.

Todd, will you do the honour of submitting a merge request?