[vtk-developers] vtk_module_autoinit in new module system

[Ben Boeckel]

On Tue, Jan 22, 2019 at 18:15:54 +0100, Elvis Stansvik wrote:
> I don't exactly understand, but I'm by no means a cmake guru, but couldn't
> the required -D flags be passed through the target interface definitions of
> the targets that VTK exports, then I could get them through just
> target_link_libraries(stuff I need)?

Brad and I had discussions about this, but no. Let's take
`vtkRenderingCore` as an example. First, you need to know what other
targets the consuming target uses (not possible with generator
expressions). Once you have that list, you need to filter down which
ones `IMPLEMENT vtkRenderingCore`, count them and join their library
names using `,`. This results in something like this:

    #define vtkRenderingCore_AUTOINIT 2(vtkRenderingOpenGL2,vtkRenderingFoobar)

This then is written to a file since MSVC doesn't support commas (or
parentheses; I forget which) on the command line. The resulting
generated `-D` flag is:

    -DvtkRenderingCore_AUTOINIT_INCLUDE=\".../vtkModuleAutoInit_${hash}.h\"

where `${hash}` is the MD5 of the set of modules being linked (to avoid
collisions when linking multiple times).

The old module system just gave you everything possible for all usages
via `VTK_DEFINITIONS`. Now, you get exactly what is necessary, nothing
more.

> Just hoping there's some way of getting rid of the double bookkeeping since
> it feels like a step back maintenancewise from a user pov if I have to
> remember to do this.

I agree that it is less handy, but there's not much to be done with
CMake features available today.

--Ben