[Dune] Reverse iterators in FieldVector

[Markus Blatt]

On Thu, Nov 26, 2009 at 06:12:12PM +0100, Oliver Sander wrote:
> Dear Dune!
> When working on FS 438 I found the following behavior of FieldVector:
> The method rbegin() returns a bidirectional iterator to the last element
> of the vector.  To get to the second-to-last element you can call 
> operator--.
> However, the standard stl behavior is to return a reverse iterator which
> needs to have operator++ called to get to the second-to-last element.
> 
> Is this nonstandard behavior intended or is it a bug that needs fixing?
> 


It is intended and all algorithms honour and rely on this behaviour.

Changing this would be a lot of unnecessary work. Besides, the
implementation of all stl containers  is a wrapper of the forward
iterator that changes its bahaviour. This is made in way that
dereferencing a reverse iterator creates a temporary copy of the
underlying forward iterator, calls operator--
on it and dereferences it.

Of course, without measurements I cannot guarantee that this is really
slower than our implementation, but I am tempted to assume, that it
is. (Maybe 7%? ;)

BTW: The current behaviour prohibits FieldVector to be a descendant of
std::array without modifying the std behaviour. (I am sorry for not
noticing this earlier, but it just was not on my mind).

Cheers,

Markus