[petsc-dev] XXXDestroy() mistaken design in PETSc

[Barry Smith]

On Feb 15, 2011, at 5:26 PM, Matthew Knepley wrote:

> On Tue, Feb 15, 2011 at 4:47 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
> 
>  In MPI one calls MPI_Comm_free(&comm) to allow the MPI implementation to set the pointer explicitly to 0 after the object is destroyed.
> 
>  In Petsc XXXDestroy() does not pass the pointer (because it seemed too unnatural to me in 1994) thus not allowing 0ing the pointer.
> 
>   Was this a bad design decision? Should it be revisited?
> 
>   Barry
> 
>  Two use cases
> 
> 1) error detection when someone tries to reuse a freed object
> 
> We catch this with other error detection. I do not think we would gain much here.

  No really. If I do MatDestroy(mat); MatMult(mat,x,y); then it is possible that MatMutl() will crash while looking around inside where mat points. If MatDestroy(&mat); zeroed mat then MatMult(mat,x,y) could do the safe test of if (!mat) nice error message.

   Barry

>  
> 2) when removing some objects from a data structure that will be used data one currently needs to do
> 
>  XXXXDestroy(mystruct->something);CHKERRQ(ierr); mystruct->something = 0;
> 
> instead of the cleaner XXXDestroy(&mystruct->something);CHKERRQ(ierr);
> 
> True, but again I do not think the win is large.
> 
>    Matt
> 
> -- 
> 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