[vtk-developers] Should VTK targets have INTERFACE_{INCLUDE_DIRECTORIES, COMPILE_DEFINITIONS} set?

[Elvis Stansvik]

Hi all,

When linking against a VTK module through an imported target, e.g

  find_package(VTK 8.0.0 REQUIRED COMPONENTS vtkCommonCore)
  ...
  target_link_libraries(myapp vtkCommonCore)

I must still manually make sure I use

  target_include_directories(myapp PUBLIC ${VTK_INCLUDE_DIRS})
  ...
  target_compile_definitions(myapp PUBLIC ${VTK_DEFINITIONS})

to get the include directories and definitions.

This is in contrast to e.g. Qt's CMake files, which makes sure targets
have INTERFACE_INCLUDE_DIRECTORIES and INTERFACE_COMPILE_DEFINITIONS
set on its targets. For example, QtSvg (from Qt5SvgConfig.cmake):

    set_property(TARGET Qt5::Svg PROPERTY
      INTERFACE_INCLUDE_DIRECTORIES ${_Qt5Svg_OWN_INCLUDE_DIRS})
    set_property(TARGET Qt5::Svg PROPERTY
      INTERFACE_COMPILE_DEFINITIONS QT_SVG_LIB)

Now, this is not really a bit inconvenience, I just have to make sure
to add ${VTK_INCLUDE_DIRS} and ${VTK_DEFINITIONS}.

However, I recently started using the libclang-based
include-what-you-use tool [1] to help sanitize my header inclusions.
The tool will make suggestions such as (taking a Qt header as example
here):

/home/estan/orexplore/insight/src/model/HoleLoader.cpp should add these lines:
#include <QFileInfo> // for QFileInfo

I noticed that for a VTK header, it'll instead suggest e.g:

/home/estan/orexplore/insight/src/model/Hole.cpp should add these lines:
#include "vtkSmartPointer.h" // for vtkSmartPointer

Notice the "" instead of <>.

I dug into include-what-you-use to find out why this is happening, and
it's because on the compile line, the Qt include directories are given
with -isystem, while the VTK include directories are given with -I,
and -isystem vs -I is what the tool uses (for lack of other
information) to determine whether to suggest the inclusion with <> or
with "".

I believe that maybe the -I flags added for VTK would become -isystem,
if they were brought in through the INTERFACE_INCLUDE_DIRECTORIES
mechanism, same as Qt.

So all this lead me to two questions regarding VTK:

1. Should VTK's installed CMake files perhaps make sure
INTERFACE_INCLUDE_DIRECTORIES and INTERFACE_INCLUDE_DEFINITIONS are
set on its exported targets?

2. If not, anyone know if there's something that could be done to make
the flags end up as -isystem flags instead of -I when using
${VTK_INCLUDE_DIRS}?

Cheers,
Elvis

[1] https://github.com/include-what-you-use/include-what-you-use