Re: Does this memory access yield undefined behaviour?

From:

SG <s.gesemann@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 8 Apr 2009 12:54:36 CST

Message-ID:

<15969fcd-7331-4b36-9602-f95033a22699@l1g2000yqk.googlegroups.com>

On 8 Apr., 15:51, Arne Mertz <n...@arne-mertz.de> wrote:

So as a conclusion, if we have two pointers of different Types
pointing to the same storage location (e.g. as a result of an
interpret_cast) and actually access that storage using the pointers
alternately, we have two alternating lifetime-cycles of Objects of
the two types.
In the presence of concurrency, having pointers to the same storage
location in different threads/processes would mean it was
indeterminate if the lifetime of that object has ended or not.
Sounds a bit like "Schr?dingers pointer".

You don't need concurrent access to get a weird behaviour. The
compiler is allowed to assume that a pointer T* and a pointer U* don't
point to the same memory location in case neither T nor U is char or
unsigned char (strict aliasing rule). Under this assumption the
compiler is allowed to reorder instructions. If you violate this
aliasing rule instruction reorderings may become observable.

Cheers!
SG

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