Re: [Boost-users] [atomic] can it bechecked that boost::atomic<T> *v = reinterpret_cast<boost::atomic<T>*>(var); is safe?

Subject: Re: [Boost-users] [atomic] can it bechecked that boost::atomic<T> *v = reinterpret_cast<boost::atomic<T>*>(var); is safe?
From: Ben Pope (benpope81_at_[hidden])
Date: 2014-09-15 08:00:40

On Monday, September 15, 2014 07:31 PM, Tim Odenthal wrote:
> Dear experts,
>
> essentially, I use this type of reinterpret_cast for doubles, and it __seems__ to work fine. I compile-time check that the sizeof(boost::atomic<T>) == sizeof( T ), but is that enough to ensure that this will always work? Should/could I check alignment issues as well? Even if the "atomic_cast" function has a (small) run-time overhead, it would be very much worth it in my setting, where I actually want to safely add to a double at some location in memory. Obviously, I want to use boost::atomic to be system/compiler agnostic, since it's supposed to eventually run with clang, gcc, (linux, OS X) as well as MS compilers, and so far we didn't see the need to move to C++11...

I'm not sure I can figure out what problem you're trying to solve.

I think the cast is probably "safe", but far less can be said of it when 
it's dereferenced.  That would depend entirely on the type of var.

Is *var actually a boost::atomic<T> in the first place?  If it is, why 
are you erasing its type?

How can you be sure there is nobody else aliasing var or *var and then 
reading or writing it?

If *var is not a boost::atomic<T>, reinterpret_cast isn't going to 
magically convert it, you have undefined behaviour.  Of course, one 
effect of UB is that it __seems__ to work, until, presumably, it doesn't.

Ben