dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

On Tue, 2010-08-03 at 19:14 +0200, Anders Logg wrote:
> On Tue, Aug 03, 2010 at 03:41:35PM +0100, Garth N. Wells wrote:
> > On Tue, 2010-08-03 at 07:51 +0200, Florian Rathgeber wrote:
> > > -----BEGIN PGP SIGNED MESSAGE-----
> > > Hash: SHA1
> > >
> > > On 02.08.2010 18:50, Garth N. Wells wrote:
> > > > On Mon, 2010-08-02 at 10:33 +0200, Anders Logg wrote:
> > > >> On Sun, Aug 01, 2010 at 06:35:22PM +0100, Garth N. Wells wrote:
> > > >>> On Sun, 2010-08-01 at 12:40 +0200, Florian Rathgeber wrote:
> > > >>>> -----BEGIN PGP SIGNED MESSAGE-----
> > > >>>> Hash: SHA1
> > > >>>>
> > > >>>> Hi,
> > > >>>>
> > > >>>> When using GenericTensor::down_cast() for a tentative down cast it would
> > > >>>> be helpful if it would throw std::bad_alloc (as any dynamic_cast would
> > > >>>> do)
> > > >>>
> > > >>> Do you mean std::bad_cast?
> > > >>>
> > > >>>> instead of invoking dolfin::error. Currently you would have to catch
> > > >>>> std::runtime_error and check what() to distinguish a failed cast from
> > > >>>> another dolfin::error, which is pretty inconvenient. Is there a specific
> > > >>>> reason why it is implemented this way?
> > > >>>>
> > > >>>
> > > >>> Not really.
> > > >>>
> > > >>> It's not too clear to me what you'd like. Would you prefer that
> > > >>> down_cast doesn't throw an error, but leave it up to the programmer to
> > > >>> check that a cast was successful, or just that DOLFIN catch a
> > > >>> std::bad_cast and print more information before throwing an error?
> > > >>
> > > >> I think the best would be if DOLFIN caught the error, then wrote an
> > > >> informative message (since it knows exactly what went wrong) and then
> > > >> threw bad_cast.
> > > >>
> > > >
> > > > Looks like there isn't much that can be done - std::bad_cast is only
> > > > thrown when casting references, not pointers (as we do internally). From
> > > > what I've read, our way of checking for a null pointer is correct.
> > >
> > > If down_cast() were implemented as casting the reference directly,
> > > dynamic_cast would throw std::bad_cast if it fails.
> > >
> > > The check for the null pointer is correct, my point is that in this case
> > > not a std::runtime_error should be thrown (which happens by calling
> > > dolfin::error), but instead the message should be printed e.g. by
> > > dolfin::warning and then a std::bad_cast thrown. That was Anders'
> > > suggestion if I got that right.
> > >
> > > In that way down_cast() could be used as a tentative cast which is not
> > > really possible if it throws the same exception that could be caused by
> > > and DOLFIN error.
> > >
> >
> > GenericTensor::down_cast now catches an exception and then throws an
> > error. It was simpler than I thought because return statements can be
> > used inside a try block (which I didn't know).
> >
> > Garth
> 
> Does this work as intended? It still looks like we catch the exception
> and let then throw a runtime error by calling error():
> 
>  catch (std::exception& e)
>  {
>    error("GenericTensor cannot be cast to the requested type: %s", e.what());
>  }
> 

Yes. What else do we want to do other than eventually throw an error? We
don't want to carry on with a bad cast. If a programmer wants to do
something fancy when a cast fails, they can use dynamic_cast directly
and catch an exception.

Garth 


> --
> Anders