dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Feb 16, 2009 at 12:39:15PM +0100, Johan Hake wrote:
> On Monday 16 February 2009 12:06:09 Anders Logg wrote:
> > On Mon, Feb 16, 2009 at 11:52:54AM +0100, Johan Hake wrote:
> > > On Monday 16 February 2009 11:31:36 Anders Logg wrote:
> > > > On Mon, Feb 16, 2009 at 10:12:21AM +0000, Garth N. Wells wrote:
> > > > > Anders Logg wrote:
> > > > > > On Mon, Feb 16, 2009 at 10:36:52AM +0100, Johan Hake wrote:
> > > > > >> On Sunday 15 February 2009 21:23:44 DOLFIN wrote:
> > > > > >>> One or more new changesets pushed to the primary dolfin
> > > > > >>> repository. A short summary of the last three changesets is
> > > > > >>> included below.
> > > > > >>>
> > > > > >>> changeset:   5701:d3661203791d9c7707695c59adbbd3a2e20a220c
> > > > > >>> tag:         tip
> > > > > >>> user:        Anders Logg <logg@xxxxxxxxx>
> > > > > >>> date:        Sun Feb 15 21:23:36 2009 +0100
> > > > > >>> files:       dolfin/function/Function.cpp
> > > > > >>> description:
> > > > > >>> Move code from Function copy constructor to assignment operator
> > > > > >>> and call assignment operator from copy constructor
> > > > > >>
> > > > > >> I liked Garth solution better.
> > > > > >>
> > > > > >>  1) A copy constructor that, just copies the Function if it has
> > > > > >>     a FunctionSpace.
> > > > > >>  2) The assignment operator works only for discrete Functions.
> > > > > >>
> > > > > >> We could add an interpolate() (or something) function that
> > > > > >>
> > > > > >>   v.interpolate(*_vector, *_function_space);
> > > > > >
> > > > > > We already have exactly such a function.
> > >
> > > Do we?
> >
> > Yes:
> >
> >   /// Interpolate function to given function space
> >   void interpolate(GenericVector& coefficients, const FunctionSpace& V)
> > const;
> 
> Can you use this to initialize your own vector if you e.g. is a user-defined 
> Function? I think we have ahd this discussion before, and Martin added such a 
> function, but removed it because it was not general enough, or am I wrong?

Yes, it should work, both for interpolation of user-defined and
discrete functions.

> > > > > >> Then the user can explicitly create a discrete function of its
> > > > > >> user-defined Function. Now the user gets this as an implicitly
> > > > > >> result of a function copy, which make litle sense to me.
> > > > > >>
> > > > > >> But that's just me :)
> > > > > >
> > > > > > I like it. Other opinions?
> > > > >
> > > > > It is neat, but I would prefer any interpolation to be more explicit
> > > > > so that it's clear what's going on. A copy should be a straight copy.
> > > > >
> > > > > Garth
> > > >
> > > > ok. I've changed it back. See if it looks ok.
> > >
> > > Now a user cannot copy a Function that is not a discrete function, which
> > > was the case before we started all this.
> >
> > Wasn't that the point? It's not possible to copy the eval() operator.
> >
> > Well it is but then it would be necessary to keep a pointer to the
> > given Function and propagate the eval call to that Function's eval.
> > That seems a bit overkill.
> >
> > > Also sometimes a copy is something different than an assignment, so it is
> > > not always meaningfull to use *this = other; in the copy constructor.
> >
> > I've found it's almost always the case that one can implement the
> > copy constructor by
> >
> >   *this = other;
> >
> > We use this in a bunch of other places, including the Mesh class.
> >
> > In which cases will it break?
> 
> I am thinking very simple here, it deals with defining a sane and complete C++ 
> interface to Function. Should we be able to copy a user-defined Function or 
> not? 

I think we should (by interpolation) but if there's otherwise
consensus we should not interpolate then it should not be possible to
assign user-defined functions since we can't or won't deal with
copying of eval().

> I thought yes, and no one came with any argument why not. Remember that this 
> came from the discussion of returning a user-defined Function by value from 
> an other function.
> 
> we are talking about
> 
>   MyFunc f(V);
>   MyFunc g(f);
> 
> not
> 
>   MyFunc f(V);
>   Function g(f);

What is the difference? Both will be possible but only the first will
make sense.

> So the logic is that a user-defined Function should stay the same if it is 
> copied and the assignment operator can only be used for discrete functions. 
> 
> If we want to be able to copy a user-defined function without initializing the 
> _vector, then the assignment operator needs to be different from the copy 
> constructor.

I still don't understand why they should be different.

-- 
Anders

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