Re: [boost] [intrusive_ptr] atomic ref counting?

From: Eric Niebler (eric_at_[hidden])
Date: 2006-04-18 12:17:27

Peter Dimov wrote:
> Eric Niebler wrote:
> 
>>Peter Dimov wrote:
>>
>>>I can't help but think that even if I did add the above, you
>>>wouldn't use it because of the virtual destructor,
>>
>>You're right, because in xpressive alone I use counted_base in no less
>>than 3 places where the virtual destructor would be needless overhead!
>>Clearly, it can't be all that uncommon.
> 
> It may be common. The point is that we need to settle on a single version of 
> boost::counted_base. The virtual destructor version does have its benefits: 
> it's easier to understand and use (CRTP can be a bit hard to swallow) and it 
> allows intrusive_ptr<counted_base> to be used as the intrusive version of 
> shared_ptr<void> and void* (with the additional benefit of a working 
> dynamic_cast.)
> 
> counted_base is somewhat bicycle sheddish and I've considered adding one as 
> documentation rather than code. But one problem with that is atomic_count 
> being in detail. :-)

I'm confident that we can settle on a single design, and we should. I'd 
rather include your counted_base with a virtual than leaving it in the 
docs where people will ignore it and it won't get used.

How about this:

template<typename Derived>
struct basic_counted_base
{
     ... my counted_base implementation here ...
     ... with no virtual destructor ...
};

struct counted_base
   : basic_counted_base<counted_base>
{
     virtual ~counted_base() {}
};

That way we get the best of both worlds.

>>The following program exposes the problem for me with vc7.1, vc8 and
>>gcc 3.4.4. If you comment out the friend functions and uncomment the
>>global free functions, the problem goes away. I don't understand it --
>>is some name look-up subtlety at play here?
> 
> I don't understand it either. Comeau/EDG compiles it. A non-template 
> counted_base doesn't have this problem, FWIW.

Confirmed. Also, the problem doesn't happen when impl in my example is a 
non-template. Looks like compiler bugs, but it's strange that both VC 
and gcc reject it with similar error messages. <shrug>

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com