[petsc-dev] Forward declarations of types?

[Jed Brown]

We currently use recursive includes everywhere, so petscdmda.h includes all
of petscao.h just so it can declare DMDAGetAO and similar. Of course most
users of (and implementation files in) DMDA do not reference AO so they
don't need to know about all the AO functions.

The normal approach to this is to forward-declare the type, so instead of

#include <petscao.h> /* includes lots of other stuff */
PETSC_EXTERN PetscErrorCode DMDAGetAO(DM,AO*);

one would write

typedef struct _p_AO *AO;
PETSC_EXTERN PetscErrorCode DMDAGetAO(DM,AO*);

in which case all three files in PETSc that actually call AO routines would
need to include petscao.h. That is arguably a good thing since it makes the
actual dependencies more explicit, and is recommended by many (mostly C++)
style guidelines.

Is this something worth considering? I think stuff like petscvec.h and
petscmat.h ends up pretty much always being needed, but petscdm.h is only
used by a handful of files in petscksp and above, for example.

It might be nice to get rarely-used stuff like petscdraw.h out of
petscsys.h.
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