[Dune-devel] Random-Access Iterators

[Jö Fahlke]

Hi!

The random-access entity iterators are now ready for general review.
https://cgit.dune-project.org/repositories/dune-grid/log/?h=feature/random-access-entity-iterators

@Carsten: are you willing to include this in 2.4, provided noone finds any
          serious issues?

A grid declares that certain entity iterators are random-access by doing

  typedef std::random_access_iterator_tag iterator_category;

in the iterator implementation class.  The facade class Dune::EntityIterator
notices this and defines the additional methods needed for random access
iterators in terms of the member functions advance(n) and distanceTo(otherIt)
of the iterator implementation class.  If your grids entity iterators do

  typedef std::forward_iterator_tag iterator_category;

or omit the member type completely, the EntityIterator will be a forward
interator, as before.

To do this, Dune::EntityIterator no longer implements the iterator methods
itself, but derives from Dune::EntityIteratorBase, which has a template
parameter category to select which type of iterator to implement.  Previously
the ancestry looked like

  EntityPointer
  EntityIterator

now it looks like (for forward iterators)

  EntityPointer
  EntityIteratorBase<..., std::forward_iterator_tag>
  EntityIterator

or (for random access iterators)

  EntityPointer
  EntityIteratorBase<..., std::forward_iterator_tag>
  EntityIteratorBase<..., std::random_access_iterator_tag>
  EntityIterator

The template argument list of EntityIterator is completely the same as before.

There is one function that has been omitted from random access iterators for
the time being, namely operator[].  There are three possibilities for the
return type of operator[]: Either return the entity by value -- but that needs
copyable entities, which are not ready yet.  Or return a reference to the
entity -- not possible, since we have nothing that stores that entity.  Or
return a proxy object -- but I'm too lazy to implement that, since we will
have copyable entities soon.  Note that instead of the (currently) invalid
expression "it[n]" you can write "*(it+n)": The + creates a temporary
iterator that holds the entity.

I also omitted implementing bidirectional iterators, since I could not come up
with a use case for iterating backwards through the grid.  If a grid declares
an iterator implementation as bidirectional, the resulting EntityIterator will
nevertheless just be a forward iterator.  It should be easy to add
bidirectional iterators, should the need arise.

Regards,
Jö.

-- 
Jorrit (Jö) Fahlke, Institute for Computational und Applied Mathematics,
University of Münster, Orleans-Ring 10, D-48149 Münster
Tel: +49 251 83 35146 Fax: +49 251 83 32729

Kiss a non-smoker; taste the difference.
-- fortune
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 828 bytes
Desc: Digital signature
URL: <https://lists.dune-project.org/pipermail/dune-devel/attachments/20140930/57f90e2b/attachment.sig>