On Sun, Sep 07, 2008 at 09:15:40AM +0200, Andy Ray Terrel wrote:
On Sun, Sep 7, 2008 at 12:41 AM, Martin Sandve Alnæs <martinal@xxxxxxxxx> 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.
Having different options for ownership of data isn't just
an interface issue, it's important for coupling dolfin code
with other code.
It's also not only Function that's affected by this issue,
you might have a PETSc Vec or Epetra_FEVector and
wish to assemble into if, so you'll need to wrap it in a
dolfin::*Vector during assembly, without giving up
ownership of the data.
As far as an interface is concerned I like having a vector hidden from
me so only having the 2nd option is fine, if all the other
functionality is working as it should. It might just be because of
all the functional programming UoC has made me teach but I really
don't like the init methods in the user interface. My preference is
to have the copy constructors and assignment operators work correctly
so that to reinitialize one just needs to do something like:
Function u;
Vector x;
u = Function(x);
I think that looks good, but the last line in your example will first
call the constructor in Function and then everything will be copied
using the assignment operator. But I guess this is not a problem if
the members are shared pointers.
I also dislike the init() functions and I try to avoid them, but very
often we need to add them in to do things like reading from file:
Function u:
File file("solution.xml");
file >> u; // init() function called in XML reader
BTW, it seems this discussion on function and dof maps has been going
on for a while and spread out over many email threads. Would this be
a good issue to keep track of on bugzilla?
-- Andy
Would that work? I feel it's easier to have a discussion on the
mailing list than on the bug tracker. The problem is that all the
times we've been discussing this, we've lost momentum after some time
and then we forget about it. Let's see if we can finish up this time.
