[boost] [smart ptr] [shared] suggestion: single pointer in object returned from make_shared

Subject: [boost] [smart ptr] [shared] suggestion: single pointer in object returned from make_shared
From: Julian Gonggrijp (j.gonggrijp_at_[hidden])
Date: 2012-12-08 23:06:56

Dear Smart Ptr maintainer(s),

I recently learned that boost::make_shared allocates the space for the 
pointee and the reference counter in one go. It makes a lot of sense, 
and I might have suggested it if it wasn't already the case.

Now I'm assuming that a shared_ptr stores two raw pointers internally, 
one to the pointee and one to the counter. I guess that would be 
necessary in the "normal" case where the pointee and the counter are 
allocated separately.

Now in the case where the shared_ptr is created by make_shared, 
ideally you'd need to store only one raw pointer because pointee and 
counter reside in the same memory object. That would be the finishing 
touch to a nonintrusive smart pointer that really performs like an 
intrusive one. To me such a thing seems highly desirable.

I realise this would require a new implementation of the shared 
pointer. Could this somehow be made to cooperate or at least coexist 
with the old shared_pointer? Would it perhaps even be an option to 
deprecate the old shared_pointer in order to entirely replace it by 
the new version in the future?

I'll be most interested to read your comments on this idea.

-Julian