dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Tue, Sep 29, 2009 at 03:07:33PM +0200, Johan Hake wrote:
> On Tuesday 29 September 2009 14:06:20 Garth N. Wells wrote:
> > Anders Logg wrote:
> > > On Tue, Sep 29, 2009 at 01:01:35PM +0100, Garth N. Wells wrote:
> > >> Anders Logg wrote:
> > >>> On Tue, Sep 29, 2009 at 12:42:54PM +0100, Garth N. Wells wrote:
> > >>>> Anders Logg wrote:
> > >>>>> On Tue, Sep 29, 2009 at 07:39:01AM +0100, Garth N. Wells wrote:
> > >>>>>> Is there a reason why the MeshFunction.get(..) functions return by
> > >>>>>> value rather then reference, e.g. it is currently
> > >>>>>>
> > >>>>>>      inline T get(const MeshEntity& entity) const
> > >>>>>>
> > >>>>>> rather than
> > >>>>>>
> > >>>>>>      inline T& get(const MeshEntity& entity) const
> > >>>>>>
> > >>>>>> Garth
> > >>>>>
> > >>>>> Yes, we have a set() function for setting the values.
> > >>>>
> > >>>> I know, but I would like to get a reference rather than copying data
> > >>>>
> > >>>>     unsigned int index = 3;
> > >>>>     const std::vector<double>& vec =  mesh_function.get(index)
> > >>>>
> > >>>> I can do this when I have the MeshEntitiy, but not when using the
> > >>>> index.
> > >>>
> > >>> Does it work if you do
> > >>>
> > >>>   &vec[0] = mesh_function.values();
> > >>
> > >> Probably, bit it's not what I want. I have a std::vector attached to
> > >> mesh entities (MeshFunction<std::vector<double> >). I want a reference
> > >> to the std::vector attached to a particular entity
> > >>
> > >>    std::vector<double> vec& =  mesh_function(my_entity);
> > >>
> > >> is fine, but
> > >>
> > >>    std::vector<double> vec& =  mesh_function.get(my_index);
> > >>
> > >> is not.
> > >
> > > What if you just add operator[] for uint?
> > >
> > >> Back to my question: Is there a reason why MeshFunction::get returns by
> > >> value and not by reference?
> > >
> > > There might be a reason. I think we introduced get/set for use in the
> > > Python interface. Perhaps Johan remembers?
>
> Well, this was before my time ;)
>
> But I see that get is used to overload the __call__ method in Python. I am
> quite sure this can be done in a nicer way. We probably had difficulties with
> extending the MeshFunctions as these are templated classes. We know how to do
> this now.
>
> I am not sure why we return by value, but it might have something to do with
> memory management in Python. However these things could be handled in the
> wrapper layer anyway.
>
> I suggest we define the C++ interface as we want it to be, then I am quite
> sure I can mimic this in Python. (starting to get bald I think ;))
>
> So whats the logic here. Should
>
>   &T operator()
>
> be used when we have the entity, and maybe
>
>   &T operator[]
>
> when we have the index, or should one of the operators be used for both cases?

I think it would be safe to overload operator[].

--
Anders


> Johan
>
> > OK, that's what I want to know.
> >
> > Garth
> >
> > >
> > >
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