dolfin team mailing list archive

[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?

-- 
Anders

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