Re: [libstdc++/61347] std::distance(list.first(),list.end()) in O(1)

[Jonathan Wakely <jwakely at redhat dot com>]

On 14/04/15 13:50 +0200, Marc Glisse wrote:
> [dropping gcc-patches]
>
> On Tue, 14 Apr 2015, Jonathan Wakely wrote:
>
> > We should also look into applying the same optimisation for
> > __gnu_debug::list, as long as we don't lose the ability to detect
> > errors like std::distance(l1.begin(), l2.end()) where &l1 != &l2.
>
> It requires its own implementation (in particular to detect 
> distance(next(it),it)). It should be quite simple. Actually, I believe 
> it can be done generically for all debug containers, since iterators 
> contain a pointer to the container: check _M_is_begin() on the first 
> argument, _M_is_end() on the second, check that their containers are 
> the same, and return size() (we would still need to disable that for 
> forward_list).
>
> I am not sure how useful it is though. By not walking the list, we 
> will always lose some verification.

Yeah, we wouldn't want to stop aborting on this by turning it into
c.size():

 std::distance(c.begin()+1, c.begin());