dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Sun, Sep 07, 2008 at 03:38:44PM +0100, Garth N. Wells wrote:
> 
> 
> Anders Logg wrote:
> > On Sun, Sep 07, 2008 at 03:29:14PM +0200, Anders Logg wrote:
> >> On Sun, Sep 07, 2008 at 03:11:51PM +0200, Martin Sandve Alnæs wrote:
> >>> 2008/9/7 Anders Logg <logg@xxxxxxxxx>:
> >>>> On Sun, Sep 07, 2008 at 08:27:41AM +0100, Garth N. Wells wrote:
> >>>>>
> >>>>> Martin Sandve Alnæs wrote:
> >>>>>> 2008/9/6 Anders Logg <logg@xxxxxxxxx>:
> >>>>>>> On Sat, Sep 06, 2008 at 04:22:09PM +0200, Martin Sandve Alnæs wrote:
> >>>>>>>> 2008/9/6 Garth N. Wells <gnw20@xxxxxxxxx>:
> >>>>>>>>> Dag Lindbo wrote:
> >>>>>>>>>> Anders Logg wrote:
> >>>>>>>>>>> There seems to be a problem (among many) with the current design of
> >>>>>>>>>>> the Function classes (see thread "evaluating higher order mesh function").
> >>>>>>>>>>>
> >>>>>>>>>>> In particular, the finite element is missing in DiscreteFunction. My
> >>>>>>>>>>> suggestion would be to just add it and let a DiscreteFunction consist
> >>>>>>>>>>> of the following four items which are always available:
> >>>>>>>>>>>
> >>>>>>>>>>>   mesh, x, dof_map, finite_element
> >>>>>>>>>>>
> >>>>>>>>>>> Is this enough, and what other issues to we need to fix?
> >>>>>>>>>>>
> >>>>>>>>>> One major issue which I just want to reiterate is ownership of data. As
> >>>>>>>>>> it stands, the DiscreteFunction may or may not be responsible for e.g.
> >>>>>>>>>> the dof vector x, depending on whether local_vector is a NULL pointer or
> >>>>>>>>>> not. Take a look at the thread "Ownership" from Garth on 06/26/2008.
> >>>>>>>>>>
> >>>>>>>>> Yes, this is a big problem and has caused me a few headaches with bugs.
> >>>>>>>>> For example, passing a user-defined Function to a function to convert it
> >>>>>>>>> to a DiscreteFunction via a projection onto a finite element basis
> >>>>>>>>> causes a problem because the FiniteElement which the projected Function
> >>>>>>>>> points to goes out of scope once the function is exited.
> >>>>>>>>>
> >>>>>>>>>> A problem related to this is initialization of the DiscreteFunction. We
> >>>>>>>>>> had a bug previously where the LinearPDE class maintained ownership of
> >>>>>>>>>> the solution vector. The only way to prevent this was to break the
> >>>>>>>>>> encapsulation of DiscreteFunction by making it a friend of LinearPDE (as
> >>>>>>>>>> with XMLFile for the same reasons). Here is some of the code that
> >>>>>>>>>> handles this initializaton today (L101 in LinearPDE.cpp):
> >>>>>>>>>>
> >>>>>>>>>>   u.init(mesh, *x, a, 1);
> >>>>>>>>>>   DiscreteFunction& uu = dynamic_cast<DiscreteFunction&>(*u.f);
> >>>>>>>>>>   uu.local_vector = x;
> >>>>>>>>>>
> >>>>>>>>>> This ain't poetry in my opinion :)
> >>>>>>>>>>
> >>>>>>>>> Indeed, this isn't nice, and there is something similar in XMLFile.cpp.
> >>>>>>>>>
> >>>>>>>>> Garth
> >>>>>>>> We should start to use std::tr1::shared_ptr. There is some support for it
> >>>>>>>> with python in swig 1.3.35, which is part of the upcoming Ubuntu Intrepid
> >>>>>>> The main issue is how we want to initialize Functions, and if one
> >>>>>>> should allow to set members.
> >>>>>>>
> >>>>>>> For simplicity, say that a Function is defined only by a Vector.
> >>>>>>> Then we have a few different situations to consider:
> >>>>>>>
> >>>>>>> 1. Function creates the Vector
> >>>>>>>
> >>>>>>>   Function u;
> >>>>>>>   Vector& x = u.vector();
> >>>>>>>
> >>>>>>> 2. Function gets the Vector
> >>>>>>>
> >>>>>>>   Vector x;
> >>>>>>>   Function u(x);
> >>>>>>>
> >>>>>>> 3. Function gets initialized with a Vector
> >>>>>>>
> >>>>>>>   Function u;
> >>>>>>>   Vector x;
> >>>>>>>   u.init(x);
> >>>>>>>
> >>>>>>> Do we want to support all of 1-3? Things become considerable easier if
> >>>>>>> we can make some simplifying assumptions.
> >>>>>>>
> >>>>>>> How visible would a shared_ptr be in the interface?
> >>>>>> A shared_ptr must be visible to the user every single place
> >>>>>> a pointer is passed around, otherwise the reference count
> >>>>>> won't be correct and we'll just have more problems.
> >>>>>>
> >>>>> So, in pseudo code, would it look something link this?
> >>>>>
> >>>>>    class DiscreteFunction
> >>>>>    {
> >>>>>      private:
> >>>>>
> >>>>>        shared_ptr<GenericVector> x;
> >>>>>
> >>>>>      public:
> >>>>>
> >>>>>        DiscreteFunction() : x(new Vector) {}
> >>>>>
> >>>>>        DiscreteFunction(shared_ptr<GenericVector> x)
> >>>>>        { x(x); }
> >>>>>
> >>>>>        shared_ptr<GenericVector> vec()
> >>>>>        {return x;}
> >>>>>    }
> >>>>> ?
> >>>>>
> >>>>> Garth
> >>>> What would the user code look like if we use shared_ptr for examples
> >>>> 1-3 above?
> >>>>
> >>>
> >>>>>> 1. Function creates the Vector
> >>>>>>
> >>>>>>   Function u;
> >>>>>>   Vector& x = u.vector();
> >>> Function u;
> >>> Vector& x = u.vector(); // Storing this Vector& for later access is unsafe.
> >>> or
> >>> Function u;
> >>> shared_ptr<Vector> x = u.vector(); // Allows keeping the Vector around
> >>> after u is destroyed.
> >>>
> >>>
> >>>>>> 2. Function gets the Vector
> >>>>>>
> >>>>>>   Vector x;
> >>>>>>   Function u(x);
> >>> Vector x;
> >>> Function u(x); // Copy vector.
> >>>
> >>> shared_ptr<Vector> x = new Vector();
> >>> Function u(x); // Copy vector pointer, x or u may be deleted without
> >>> the other getting in trouble.
> >> I don't think the first option is what one might expect, and I don't
> >> think the second example looks very nice.
> >>
> >> We initialize Functions with a Mesh all the time and it would then be
> >> either very expensive to copy the mesh every time we create a Function
> >> from it (and one usually creates many functions on the same mesh), or
> >> we would have to write "shared_ptr" and "new" every time we used a
> >> Mesh.
> >>
> >> Isn't there another option? I don't like the all the flags we have now
> >> like is_view, local_vector, etc, but this looks worse.
> > 
> > Is there a way to increase the count for a shared_ptr?
> > 
> > If there is, say a member named increase_ref(), we could do
> > 
> >   class DiscreteFunction
> >   {
> >   public:
> > 
> >       DiscreteFunction() : x(new Vector) {}
> > 
> >       DiscreteFunction(GenericVector& x) : x(x);
> >       {
> >           x.increase_ref(1);
> >       }
> > 
> >       DiscreteFunction(shared_ptr<GenericVector> x) : x(x) {}
> >       { x(x); }
> > 
> >       shared_ptr<GenericVector> vec()
> >       {return x;}
> > 
> >   private:
> > 
> >       shared_ptr<GenericVector> x;
> > 
> >   };
> > 
> > Then it would be possible to do
> > 
> >   Vector x;
> >   Function u(x);
> > 
> > as we can now and the Function u would know that someone else is
> > responsible for deleting the data.
> > 
> > Then if one writes code where the Vector goes out of scope, one must
> > use a shared_ptr, but not otherwise. We would not force everyone to
> > use shared_ptr all the time.
> > 
> 
> I think that having two options would lead to confusion.
> 
> On a another note, I don't really like the above code anyway. We could 
> avoid this in many cases by adding the constructor
> 
>    Function::Function(Mesh& mesh, Form& form, uint i = 1);
> 
> and letting Function create the Vector. Function already has a member 
> function for accessing the underlying Vector.
> 
> Garth

That's what I suggested earlier. It would simplify many things if we
always know who owns what. We could assume that the Function never
owns the Mesh but always owns the Vector (for discrete functions).

If someone has a Vector and wants to create a Function from it, then
just do

  Vector x;
  Function u;
  u.vector() = x; // Copy data

-- 
Anders

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