[petsc-dev] Preprocessor hell: #define VecType

[Barry Smith]

On Sep 29, 2012, at 11:03 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Fri, Sep 28, 2012 at 9:24 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
>   I don't like having two typedefs
> 
>   I tried
> 
> typedef const char* VecType;
> PETSC_EXTERN PetscErrorCode VecSetType(Vec, const VecType);
> 
> ^^^ Note that this const is superfluous.

    But is it harmful? (Will it every produce a compiler complaint?)   It is nice to have const on sets and not a const on gets

   Barry


>  
> PETSC_EXTERN PetscErrorCode VecGetType(Vec, VecType *);
> 
> then everything compiled fine except I need to change in a bunch of places
> 
> ierr = PetscStrallocpy(ctype,(char**)&da->vectype);CHKERRQ(ierr);
> 
>   I can live with this interface (that is requiring some casting when manipulating as strings). Does anyone see problems with this approach?  I know I was opposed to this before but I've come around.
> 
> 
> 
>    Barry
> 
> 
> 
> 
> On Sep 28, 2012, at 6:16 PM, Karl Rupp <rupp at mcs.anl.gov> wrote:
> 
> > Hi,
> >
> >>     The problem is we/people may want to "build" XXXType values on the fly with string operations like strcpy, strcat etc. You cannot do that into a const char*, thus we/people would have to declare the place they build things as char* instead of XXXType and that is "unnatural".  Keep digging; we all agree with you that it would be good to get rid of the #define.
> >>
> >>     I view this problem as a slight "flaw" in typedef, but perhaps C typedef has a solution?
> >>
> >>    Barry
> >>
> >
> > okay, this explains the 'why', thanks. :-)
> >
> > I've played a bit with the options we have and finally came up with the following snippet, which  resembles the XYZSetType() and XYZGetType() functions currently in use:
> >
> > #include "stdio.h"
> > #include "stdlib.h"
> >
> > #define VecType   char*
> > #define VECSHARED "shared"
> >
> > int SetType(const VecType v){
> > printf("%c\n", v[0]);
> > return 0;
> > }
> > int GetType(const VecType* v){
> >  static char bla[] = "bla";
> >  *v = bla;
> >  return 1;
> > }
> >
> > int main(int argc, char **argv){
> >  const char test1[] = "test1";
> >  char test2[] = "test2";
> >  const char* test3 = "test3";
> >  VecType test4 = (VecType)malloc(5*sizeof(char));
> >  const VecType test5 = test4;
> >
> >  test4[0] = 'a';
> >
> >  printf("-- set --\n");
> >  SetType(test1);
> >  SetType(test2);
> >  SetType(test3);
> >  SetType(test4);
> >  SetType(test5);
> >  SetType(VECSHARED);
> >
> >  printf("-- get --\n");
> >  /*GetType(&test4);  <-- does not compile with #define
> >  printf("%c\n", test4[0]); */
> >  GetType(&test5);
> >  printf("%c\n", test5[0]);
> >
> >  return 0;
> > }
> >
> > I note the following
> > ==> It compiles cleanly
> > ==> GetType() expects a pointer to pointer to const. Well, this is probably not overly intuitive, as this suggests that the internal representation is a pointer to const char, while it is actually a char* (see petsc-private/petscimpl.h if I'm not mistaken...). Moreover, in order to call the getter function, one needs to write
> >  const VecType type;
> >  GetType(&type);
> > (see src/vec/vec/impls/nest/vecnest.c) or even the undesired
> >  const char *tname;
> >  *ierr = GetType(&tname);
> > (see src/vec/vec/interface/ftn-custom/zvecregf.c). Hence, while people will usually expect to instantiate a VecType and pass it to GetType(), they have to actually instantiate a 'const VecType' and pass it to GetType in order to get it modified appropriately (huh?).
> >
> >
> > Now let's consider the options we have:
> > a) typedef char* VecType;  no longer compiles cleanly, as Barry pointed out.
> > b) typedef const char* VecType;  imposes usability restrictions and would hence break a lot of user-code.
> > c) Keep it as-is. This compiles cleanly, yet the GetType() issue is not intuitive. Also, the Sword of Damocles (here: the preprocessor define) is a constant threat to other code...
> > d) Consider the following modification: Instead of a single preprocessor-define, use
> >  typedef       char* VecType;
> >  typedef const char* VecType_;
> > and use the function headers
> >  int SetType(const VecType_ v);
> >  int GetType(VecType * v)
> > This allows for a more intuitive const-correctness, i.e.
> >  VecType type; GetType(&type);
> > and gets rid of the C preprocessor issue. Also, the reason for the trailing underscore can be explained rather nicely in the documentation and should not cause confusion: Even if a user writes
> >  VecType_ new_type = "shared"; SetType(new_type);
> > the code remains valid.
> >
> > So, even if option d) is still not the cleanest solution we could dream of, I think it is preferable over the current state.
> >
> > Best regards,
> > Karli
> >
> >
> 
>