[petsc-dev] questions on new include organization

[Jed Brown]

On Wed, Feb 13, 2013 at 2:14 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
> *  There are other petscxxx.h include files, but generally the user need
> only include petscPACKAGENAME.h though Jed seems to have broken this model
> with some subclasses like petscdmda.h petscpcmg.h etc not being included by
> petscdm.h and petscpc.h (hmm this is both good and bad) not including a
> bunch of stuff the user doesn't need or specifically request is good but on
> the other hand the user has to specifically include it (increased learning
> curve). So to use PCMG functionality directly one must include petscpcmg.h
> directly but to use field split they don't need petscpcfieldsplit.h this
> can lead to user exasperation (and mine)!
>

My justification for splitting out the DM implementation headers was that
KSP/SNES/TS depend on the DM interface, but should not (there may be a
couple places that cheat) depend on the DMDA/DMPlex/etc interfaces. I would
say that any include of petscdmxxx.h outside of dmxxxsnes.c is a bad
dependency. The user, on the other hand, always depends on the
implementation, so they should include petscdmxxx.h.


>
> **  Note we don't do this for sub sub classes, KSPLGMRES should be
> KSPGMRESL etc. pity we should have done it right.
>

I see the appeal of hierarchies, but I find them rigid and inconvenient. I
definitely prefer object models (interface and implementation, but no
implementation inheritance). The only reason for implementation inheritance
is code sharing, but I think that's frequently a false economy and comes
back to bite us in brittleness. If we had a lightweight way to selectively
forward methods, we could break out the sharable part into an internal
object with well-defined methods and avoid the piggy-back overriding of
methods (implementation inheritance).
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