------- Comment #157 from rguenther at suse dot de 2007-05-24 10:33 -------
Subject: Re: [4.0/4.1/4.2/4.3 Regression] placement
new does not change the dynamic type as it should
On Thu, 24 May 2007, gdr at cs dot tamu dot edu wrote:
>
>
> ------- Comment #156 from gdr at cs dot tamu dot edu 2007-05-24 10:29 -------
> Subject: Re: [4.0/4.1/4.2/4.3 Regression] placement new does not change the
> dynamic type as it should
>
> "rguenther at suse dot de" <[EMAIL PROTECTED]> writes:
>
> [...]
>
> | > I brought in the union example to point of a fundamental problem with
> | > this issue. I have been following the discussion without saying much,
> | > until I realized that the interpretation Mark was offering is a
> | > redefinition of the C++ object model that conflicts with the current
> | > standard text. That was the point of the union example. In the
> | > example
> | >
> | > void f(int* p, double* q) {
> | > *p = 42;
> | > *q = 3.12;
> | > }
> | >
> | > All we know is that after the store to *p, the object there will have
> | > type int (if it did not already have one). Similarly, for the store
> | > to *q, the object there will have type double. Can the stores be
> | > rearranged? Under the current C++ rules (which were inherited from
> | > C90, and not C99) "yes" if we know that the objects are distinct.
> | > Can we infer the disjoinctness from the types? "Not always" under
> | > current C++ rules for union, and in this specific case, the answer is
> | > "no".
> |
> | Right, current C++ rules forbid exchanging the stores. But I read it
> | as it is because the stores may start lifetime of a different
> | dynamically typed object on the same memory location which p and q
> | may point to.
>
> Which is equivalent to having the union, in all aspects.
>
> Now, if I understand your argument below correctly, you are saying
> that even if we fixed the union rules for C++, we may still be facing
> the same problem because assignment to POD objects does not just mean
> that the object was there before, but that we are actually starting a
> new one. Do I understand your argument correctly?
Yes. For non-PODs I'm not sure if it is allowed to start object
lifetime via assignment rather than only by placement new.
Richard.
> | Whether there is a union "placed" at this memory
> | location is irrelevant. I read the standard as if the above is
> | equivalent to
> |
> | *(new (p) int) = 42;
> | *(new (q) double) = 3.12;
> |
> | which as long as we cannot prove that p does not point to the
> | same memory location as q means that we cannot reorder the stores.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29286
