[petsc-users] check if object was created

[Dominik Szczerba]

One little clarification:

> Vec vec = 0;
> VecCreate(..., &vec);
> VecDestroy(&vec);

This causes a compilation error for (only) VecDestroy, the only way to
avoid is to say:

VecDestroy(vec);

This confuses me, be cause the docu says like you that a pointer is
expected. I see that e.g. Mat expands to _p_Mat* but that is not the
same... ???

Dominik