On Mon, Sep 08, 2008 at 11:53:09PM +0200, Johan Hake wrote: > On Monday 08 September 2008 21:45:27 Martin Sandve Alnæs wrote: > > 2008/9/8 Anders Logg <[EMAIL PROTECTED]>: > > > On Mon, Sep 08, 2008 at 11:12:14AM +0200, Martin Sandve Alnæs wrote: > > >> 2008/9/8 Johan Hoffman <[EMAIL PROTECTED]>: > > >> >> 2008/9/8 Dag Lindbo <[EMAIL PROTECTED]>: > > >> >>> 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? > > >> >>> > > >> >>> I'm not sure I agree that the dof map and finite element should be > > >> >>> owned by the discrete function. There was a great suggestion from > > >> >>> Martin, in a thread "Abstraction idea" from 06/05/2008, to create a > > >> >>> class FunctionSpace where the mesh, element and dof_map(s) are > > >> >>> aggregated. Citing Martin: > > >> >>> U = FunctionSpace(mesh, dofmapset, form, 0) # or something similar > > >> >>> u = Function(U) > > >> >>> v = Function(U) > > >> >>> > > >> >>> This seems a solid approach to me since it would provide a way of > > >> >>> encapsulating the mathematical formulation of the problem, which is > > >> >>> more or less const and likely to be reused by many discrete > > >> >>> functions in a solver. > > >> >>> > > >> >>> It seems to me that there is an obvious risk that a lot of redundant > > >> >>> initialization would occur if all discrete functions should own > > >> >>> their own elements and dof maps. There seems to be consensus that > > >> >>> the mesh should be "global" for efficiency reasons, so why not treat > > >> >>> the function space the same way? > > >> >>> > > >> >>> Is there a problem with an approach where the funciton _always_ owns > > >> >>> the vector and _never_ owns the function space (and mesh)? A very > > >> >>> strict design would avoid shared/smart pointers, provide a > > >> >>> comprehensible user interface and probably help the parallellization > > >> >>> effort. > > >> >>> > > >> >>> /Dag > > >> >> > > >> >> If the Function always owns the vector, there are cases you'll have > > >> >> to make unneccessary copies of a vector, in particular such scenarios > > >> >> may occur when trying to combine dolfin with something else. > > >> >> > > >> >> If the Function never owns the function space, it must always be > > >> >> constructed explicitly by the user. This may not be a bad thing. > > >> >> However, if the Function is loaded from a file, nobody owns the > > >> >> FunctionSpace. > > >> > > > >> > Conceptually, I agree with Dag (and Martin?) that it is natural to > > >> > have global function spaces. And if the explicit construction of such > > >> > spaces can be made simple, it may not be a bad thing but a natural > > >> > part in setting up the mathematical problem. And I do not really like > > >> > that functions should be initialized from a form, which defines an > > >> > equation. > > >> > > > >> > I think one idea was to not force less mathematically oriented users > > >> > to worry about function spaces. I guess there are (at least) 2 types > > >> > of functions: (i) functions part of the form, and (ii) functions not > > >> > part of the form, but used in pre/postprocessing etc. > > >> > > > >> > For (i) it may be natural to construct the function space from the > > >> > form, and for (ii) it may be convenient in some cases, but it is not > > >> > really obvious that this is the best solution. > > >> > > > >> > Maybe an explicit construction of a function space can come with a > > >> > default, such as a nodal basis of piecewise linears? > > >> > > > >> > /Johan > > >> > > >> So: > > >> FunctionSpace V(mesh); > > >> Function f(V); > > >> gives a function f on piecewise linears? > > >> That's ok with me. > > >> > > >> > > >> About ownership, I think the only both robust and intuitive solution > > >> is that an object never should store a reference (or regular pointer) > > >> to another object. But as long as we are aware of the cost of doing > > >> this and state it clearly in the documentation, I'm ok with keeping > > >> e.g. the Mesh references like we do. > > >> > > >> Any time object A stores a reference to object B, the user must > > >> take care that the lifetime of B exceeds the lifetime of A. There > > >> are no exceptions to this. This puts some real limitations on the > > >> way the user must structure his program, e.g. he must sometimes > > >> (often?) keep objects around longer than they're explicitly needed. > > >> > > >> This may be a good thing, since it forces the user to think about > > >> dependencies and object lifetimes, and the objects in question > > >> use some memory. > > > > > > I think this is ok. There are many ways to create a segfault in C++. > > > If you program in C++, you will have to think about memory. > > > > > >> But if we use references instead of shared_ptr, > > >> we should never have default values: > > >> - A Function has a reference to a Mesh, which is ok since > > >> it's always created outside. > > >> - If a DiscreteFunction is to have a reference to a Vector, or a > > >> Function is to have a reference to a FunctionSpace, it cannot > > >> create its own without adding memory management code. > > >> > > >> Every place we accept these limitations and requirements of how > > >> the user structures his programs, we can use references and be > > >> done with it. But don't think that the pretty syntax means the user > > >> doesn't have to think about memory management, since all the > > >> responsibility for memory management (object destruction order) > > >> is in fact placed on the user, and errors from the users side will > > >> lead to invalid references we cannot detect and segfaults. > > > > > > I want the syntax to be simple and pretty, but I don't necessarily > > > want to hide the user from problems that are part of the design of > > > C++. It isn't Python or Java. You should be expected to know what > > > you are doing. :-) > > > > It's not only about knowing what you're doing. It forces very > > hard restrictions on the design/flow of your program, which can > > > > 1) Be a major source of bugs in nontrivial apps (see Garths email), which > > are not locally visible because they depend on the global program flow. > > > > 2) Make it impossible to initialize e.g. Function in e.g a file reader, > > since the caller of the file reader would need to get the objects > > Function depends on. This is not limited to file readers, but is > > a recurring pattern in nontrivial apps. > > > > If we want to use dolfin or want dolfin to be used in apps that > > are more complicated than the traditional "read input, compute > > something, output something" app, these restrictions become > > a larger problem. > > > > > Anyway, I like the idea about having a FunctionSpace class which > > > several Functions may share. The problem we need to solve is > > > reading from file: > > > > > > FunctionSpace V(mesh); > > > Function u(V); > > > file >> u; > > > > > > The last line just fills out the data in both u and V. > > > > > > This will lead to side effects as V might be changed when doing > > > > > > FunctionSpace V(mesh); > > > Function u(V); > > > Function v(V); > > > file >> u; > > > > > > V will be changed, both for u and v. In fact, the mesh will also be > > > changed. > > > > > > The best thing would be if we could do > > > > > > file >> (mesh, V, u); > > > > This is _exactly_ the kind of issue that smart pointers solve. > > > > Btw, I tried to search the swig documentation for shared_ptr, and > > found nothing... > > I don't know what exactly they mean by "shared_ptr support". > > It seems to be a set of typemaps that should kick in at the "right places". > They are defined in > > <Lib/python/boost_shared_ptr.i> > > and used very rudimentary in > > <Examples/test_suite/li_boost_shared_ptr.i> > > in the source tree of the 1.3.36 release. It seems that it is not specific > for > boost::share_ptr but should also figure out tr1::shared_ptr > > I think: > > %include <boost_shared_ptr.i> > > at the appropriate place should do the trick. > > Johan
I was about to start sketching on a FunctionSpace class when I realized that this class might look different for different types of Functions. In particular, some functions (DiscreteFunctions) need to have a dof map and a finite element, while this is not needed for other functions. This means that we might need to duplicate the hierarchy of different function classes in a number of different function space classes. I see two other options: 1. Require that a function space consists of mesh, finite_element, dof_map and pick a suitable (trivial) dof map for example constant functions. 2. Implement a number of different function space classes but have a single Function class which holds data but where each function call is passed on to the specific type of function call in the corresponding function space class. -- Anders
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