gridVariables is usually defined in the main filevtkWriter is assumed to be of type Dumux::VtkOutputModule or Dumux::IO::OutputModule. The data field container needs to outlive the vtkwriter object: volVars is the argument of a lambda function and does not need to be defined before this call: <module_dir>: <absolute_build_directory_path>: build-cmake/: dumux/build-cmake/doc/doxygen/html/index.html and opened via a browser.Build and run the test suite of DuMux. In build-cmake/: 
extract_as_new_module.py: CMakeLists.txt file for a test, options for [BUILD_TYPE]: Debug, RelWithDebInfo: -> for pointers and . for references when calling functions on objects.gridGeometry->gridView() vs. gridGeometry.gridView()