dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

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.

-- 
Anders


> > >> 3. Function gets initialized with a Vector
> > >>
> > >>   Function u;
> > >>   Vector x;
> > >>   u.init(x);
> 
> Function u;
> Vector x;
> u.init(x); // Copy Vector.
> 
> and/or
> 
> Function u;
> shared_ptr<Vector> x = new Vector();
> u.init(x); // Copy pointer.
>

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