Subject: Re: [boost] interest in a stateful pointer library?
From: Peter Dimov (lists_at_[hidden])
Date: 2017-10-09 14:42:46
Hans Dembinski wrote:
> > Out of curiosity, what's the specific problem that was pointed out? What
> > behavior is undefined?
>
> I was pointed to this answer on SO:
> https://stackoverflow.com/questions/34737737/relation-between-numeric-representation-of-memory-address-and-alignment
>
> I am relying on two things:
> - memory addresses map trivially to consecutive integers
> - the address of aligned memory allocated by Boost.Align always converts
> into an integer that is a multiple of the alignment value
>
> Neither seems to be guaranteed by the standard. The conversion from memory
> address to integer is an implementation detail, the SO answer gives
> examples of "exotic" mappings which are conceivable (not that there is any
> machine out there which actually does it this way).
As far as I can see, you're storing an uintptr_t. This is not undefined. The
only thing you're missing is an ASSERT that the tag_mask part is 0 after the
reinterpret_cast. Yes, this is not guaranteed to work in principle, but if
it does, it's not undefined.
Your converting constructor seems broken though. Even if we put aside the
problem that destroying U* is not the same as destroying T*, you have to
convert to T* first, then to U*, then back to uintptr_t. (And assert here as
well, as the new value is no longer guaranteed to have a tag_mask of 0.)
Stepping back a bit, I would have expected a tagged_ptr to be a low-level
component that just adds tag bits to a pointer, rather than being
responsible for allocating and destroying objects and arrays. Perhaps the
way you've implemented it makes it more useful for certain scenarios though,
I'm not sure.