[Trilinos-Users] Teuchos MpiComm

[Bartlett, Roscoe A.]

> Something this simple should not be so hard to do.  Can we introduce a
> helper function in Teuchos?

I am adding one now that will allow you to do:

void someFunc(const Teuchos::Comm<int> &comm)
{
  MPI_Comm  rawMpiComm = getRawMpiComm(comm);
  ... use rawMpiComm and forget it ...
  return;
}

In the above example you are assuming that the underlying implementation is Teuchos::MpiComm.  If that is not true, it will throw an exception.  Also, be aware that the lifetime of the returned MPI_Comm is not guaranteed.  It can go away at any time because the client is not participating in reference counting of the underlying MPI_Comm object.

Note that in general, when you use object oriented programming and object composition in C++ (by the book), you will have object hierarchies and object composite stacks.   If you are in code that works with the base interfaces and you if you want to grab an embedded object in a derived type, you *always* have to dynamic cast, then call access functions (and sometimes several times if it is a deep stack of interfaces and objects).   Where this gets "complicated" is combination the explicit dynamic casting (i.e. rcp_dynamic_cast), the memory and lifetime management (i.e. RCP), and the non-object behavior of MPI_Comm (i.e. OpaqueObject).  This is C++ and that is what you have to do to write safe flexible interoperable software.     Experienced C++ programmers are very comfortable with this approach (because that is what you have to do in C++).

But we can (and do in many case) provide non-member helper functions for common access patterns that do this dynamic casting for you.

If we want everything to be simple, we would just use Python for everything.  Python is weekly typed so there is no need for dynamic casting.  Python has reference counting built in so there is no need for RCP classes.  The problem is that Python is slow, Fortran is not a general purpose modern programming language, and C is too low level and no help with management memory, and so here we are back to C++.  Computational science is in a very bad place right now all things considered.

Cheers,

-Ross


> On 12/23/14, 12:49 PM, "Bartlett, Roscoe A." <bartlettra at ornl.gov> wrote:
> 
> >> > Teuchos::RCP<matrix_t> A...;
> >> >
> >> > //doesn't work:
> >> > Teuchos::RCP< const Teuchos::MpiComm< int > >  tr_comm = m_A-
> >> > >getComm();
> >> >
> >> > //works, but I don't have an MpiComm:
> >> > Teuchos::RCP< const Teuchos::Comm< int > >  tr_comm = m_A-
> >> >getComm();
> >> >
> >> > //fails because I don't have an MpiComm
> >> > Teuchos::RCP<const Teuchos::OpaqueWrapper<int> > cmm =
> >> > tr_comm->getRawMpiComm();
> >>
> >> Teuchos::RCP<const Teuchos::OpaqueWrapper<int> >  cmm =
> >>    rcp_dynamic_cast<Teuchos::MpiComm<int> > (tr_comm, true)-
> >> >getRawMpiComm();
> >
> >Forgot the const.
> >
> >Teuchos::RCP<const Teuchos::OpaqueWrapper<int> >  cmm =
> >   rcp_dynamic_cast<const Teuchos::MpiComm<int> > (tr_comm,
> >true)->getRawMpiComm();
> >
> >NOTE: You should not need Teuchos:: before the rcp_dynamic_cast due to
> >ADL.  The "true" is so that if the dynamic cast fails, you get a good
> >error message and not just a null pointer.
> >
> >-Ross
> >
> >
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