Re: [Boost-users] [intrusive_ptr] Counting implementation

From: Alexander Gutenev (gutenev_at_[hidden])
Date: 2008-08-01 02:37:12

> Overhead of counter and deleter is exactly the same for both shared_ptr 
> and
> intrusive_ptr - one per object (the fact that intrusive_ptr doesn't 
> support
> Deleter at the moment is beside the point IMO - it should). Slight 
> difference is
> that former is require additional new call.

Additional new call is not very little thing. Extra heap operation have its 
cost. Not sure for other platforms, but with VS2005, new is roughly malloc, 
malloc is roughly HeapAlloc, and Windows heap have overhead like rounding up 
to multiply of 16 bytes per HeapAlloc and using additional 16 bytes per 
HeapAlloc.

The reason for polymorphic deleter in shared_ptr is, IMHO, that additional 
heap object is allocated anyway, so here polymorphic deleter is almost free 
sugar.
For unique_ptr deleter is non-polymorphic, unique_ptr does not need 
additional object by itself, so here polymorphic deleter would be an 
expensive thing.
For intrusive_ptr polymorphic deleter is too expensive too. As for 
non-polymorphic - this can be controlled by intrusive_ptr_add_ref 
implementation.
Also, there's an alternative for custom non-polymorphic deleter: member 
operator delete. For PODs or not editable classes it cannot be added, but 
then inheritance from intrusive_pointee_base cannot be added too. If class 
is yours, why not write operator new/operator delete for it ?