[petsc-users] Putting petsc in a namespace

[Jed Brown]

Florian Lindner <mailinglists at xgm.de> writes:

> If a user of my class library does an #include "mypetsc.h" he should not get all the petsc symbols in his global namespace, e.g.
>
> #include "mypetsc.h"
>
> Vector v; // is an object of my class lib
> Vec pv; // should not work, since I do not want petsc symbols in my global namespace
> petsc::Vec pv; // fine, since petsc symbols are contained in namespace petsc. But this does not seem to be possible.
>
> petsc::Vec = v.vector //  vector of type petsc::Vec is a public member of Vector. That's why "mypetsc.h" needs to #include "petscvec.h". Because of that an #include "mypetsc" imports all the symbols of "petscvec" into anyone wanting to use Vector.

In your first email, you wrote:

  #include "petsc.h"
  #include <petscksp.h>  // if the user wants he can import parts of petsc 

This is trying to include the same file twice, which you cannot do.  You
can wrap a namespace around a PETSc header, but it will break if the
user includes a PETSc file directly either before or after, and macros
like ADD_VALUES, NORM_2, PETSC_VIEWER_STDOUT_WORLD, etc., will not have
the petsc:: namespace.

I think that your wrapper class with public members provides no value
and will be more difficult to use.  At least this has been the case in
every instance I have seen, and almost everyone that tries later
concludes that it was a bad idea and abandons it.  What value do you
think it provides?  It's not encapsulation because the implementation is
public and it doesn't break any dependencies because the header is still
included.  So it's mostly an extra layer of (logical) indirection
providing only a minor syntax change that will not be familiar to other
users of PETSc.  What value is that?


Now if you mean to request that all PETSc symbols be unified in the
(C-style) Petsc* namespace instead of occupying a handful of others
(Vec*, Mat*, KSP*, etc.), we know that it's the right thing to do and
have been holding off mainly to limit the amount of changes that we
force users to deal with in order to upgrade.
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