[petsc-dev] Preprocessor hell: #define VecType

[Matthew Knepley]

On Fri, Sep 28, 2012 at 3:17 PM, Karl Rupp <rupp at mcs.anl.gov> wrote:

> Hi,
>
>
> >       The reason we use a #define instead of typedef is because of
>
>> (...)
>>
>>
>> We really want to be able to const the beast in many places and I
>> couldn't get it to work with typedefs in a way natural for users.  If you
>> can come up with a clean natural way to handle the const we can switch.
>>
>
> Ok, I see. Let me be a bit persistent then and hunt for occurrences of
> 'VecType' without 'const':
>
> $> grep -r "VecType" *.h | grep -v "const VecType"
> petscvec.h:.seealso:  VecCreate(), VecType, VecSetType()
> petscvec.h:    VecType - String with the name of a PETSc vector or the
> creation function
> petscvec.h:#define VecType char*
>
>
> Alright, so
>  typedef const char * VecType;
> should work here. Let's check for MatType:
>

Unfortunately no, this does not work, or we would have done this.

   Matt


> $>grep -r "MatType" *.h | grep -v "const MatType"
> petscmat.h:.seealso:  MatCreate(), MatType, MatSetType()
> petscmat.h:    MatType - String with the name of a PETSc matrix or the
> creation function
> petscmat.h:#define MatType char*
> petscmat.h:.seealso: MatGetFactor(), Mat, MatSetType(), MatType
> petscmat.h:    Notes: MATMFFD is a specific MatType which uses the MatMFFD
> data structure
>
>
> So
>  typedef const char * MatType;
> should also be fine here as well. Repeating the game for all PETSc-types
> with the following bash-beast (i.e. repeat the procedure above for every
> #define XYZType char*):
> for t in `grep -hr "Type char" *.h | sed '/^#define */!d; s///;' | sed '/
> char\*/!d; s///;'` ; do   grep -r "${t}" *.h | grep -v "const ${t}"; done
>
> one finds only a single function which is not using a 'const' in front of
> any *Type defined via the preprocessor:
> PETSC_EXTERN PetscErrorCode MatColoringRegister(const char[],const
> char[],const char[],PetscErrorCode(*)(Mat,**MatColoringType,ISColoring
> *));
> Now, assuming that 'const' was simply forgotten here, what about
> typedef-ing everything to 'const char *' then? It shouldn't be too much of
> a painful day...
>
>
>
>  2)   Having Vec but PetscVecType would be terribly inconsistent mess.
>> Better to switch everything in one painful day.
>>
>
> In the context of what Matt said initially (of which I wasn't aware and I
> wouldn't have expected a general PETSc-type to be defined via a
> preprocessor define), it is indeed inconsistent.
>
> Best regards,
> Karli
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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