CGAL users discussion list

[Julien Demouth <>]

Bernd Gaertner a écrit :
> Julien Demouth wrote:
> > Hi all,
> >
> > Maybe it is a question which has already been raised (I've seen no 
> > sign of this in the archive of the list) but why does the CGAL::Random 
> > documentation page differ from its implementation ?
>
> In which respect does it differ?

The implementation of Random class does not contain the type 
Random::State and member functions save_state and restore_state. 
Actually, it does not use internal states and does not respect the 
sentence "Different streams are independent of each other" [CGAL online 
doc, Ref. Manual of class Random, line 3 of Sec. Definition].

> > Is there any plan of designing a "better" Random class ? 
>
> If you mean the fact that you get the same sequence when you use the 
> same seed, then I don't think it's bad. After all, it must be possible 
> to reproduce the "random" data that for example lead to a failing run of 
> an algorithm.

I agree, having the same sequence when you use the same seed is a good 
thing. The problem I wanted to emphasize is that Random instances are 
not independent. Thus, if you create a new Random instance (as it is the 
case when you create an instance of Triangulation_hierarchy_3), it 
reinitializes the internal state of _all_ your Random instances. To make 
it more concrete, I had an instance of Random to generate random points 
but each time I created a new Triangulation_hierarchy_3, the internal 
state used by my Random instance was modified. It took me some time to 
understand the problem.

Best,
Julien