WriteStlToString

gistfile1.cpp

    // Get the polyData object to write
    vtkSmartPointer<vtkPolyData> polydata = polyData();

    vtkSmartPointer<vtkSTLWriter> stlWriter =
      vtkSmartPointer<vtkSTLWriter>::New();

  #if VTK_MAJOR_VERSION <= 5
    stlWriter->SetInput(polydata);
  #else
    stlWriter->SetInputData(polydata);
  #endif

    // Set writer options
    stlWriter->SetFileTypeToBinary();
    stlWriter->WriteToOutputStringOn();

    // Write the file
    stlWriter->Write();
    char* output = stlWriter->GetOutputString();
    int outLen = stlWriter->GetOutputStringLength();

    return QByteArray(output, outLen);

I don't understand that output==nullptr and outLen==0
while writing the same polydata object to a file works like a charm.