Using std::set but need to change an element

[Gianfranco Berardi <gberardi@xxxxxxxxxxx>]

I found some code at http://www.gphysics.com to do "quick and simple" 
physics, but I find that the code was written in VC++.  I've managed to 
get it to mostly compile in g++ 4.0.4, but I'm having trouble with the 
use of std::set.

The code defines an Arbiter as the decision-maker between two Body 
objects.  It keeps the Arbiter objects in std::set<Arbiter> arbiters.

It can find out if an arbiter already exists in the set, but then the 
code wants to run some operations on each arbiter.  None of the 
operations are const, and they can't because they change the members.

The problem is that the ArbIter is a typdef to set<Arbiter>::iterator, 
which in g++ just ends up being a const_iterator anyway.  It makes sense 
since a set is supposed to have immutable members.

Then I found 
http://www.velocityreviews.com/forums/t287950-which-type-should-quotstdsetbegin-constquot-return.html 
which seems to suggest that iterator shouldn't be implemented as 
const_iterator.  Is g++ wrong and VC++ correct in this regard?

If not, how would I do something like the following:

for (ArbIter arb = arbiters.begin(); arb != arbiters.end(); ++arb)
{
  (*arb).PreStep(inv_dt);
}

Do I really need to change Arbiter's members to mutable?  I was thinking 
about erasing the arbiter from the set, using const_cast, and then 
inserting it again, which should be fine except that using const_cast 
makes me nervous.  I figure there might be a better solution that is 
currently escaping me.  Then again, it only makes me nervous because 
I've never used it and I always read that I shouldn't.  Perhaps this is 
one of those situations when its use is valid?

Thank you for your time,
Gianfranco

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