is portable aliasing possible in C++?

[Richard Biener]

On Fri, Sep 5, 2014 at 4:19 PM, Jason Merrill <jason@redhat.com> wrote:
> On 09/04/2014 07:11 PM, Jonathan Wakely wrote:
>>
>> Placement new might work with GCC in practice as long as the buffer is
>> correctly aligned and the type being constructed does not have
>> non-trivial initialization. However, my reading of the standard is
>> that after the placement new, if the object's members are not
>> initialized then they have indeterminate values (not the values that
>> were at those memory addresses already, even though that's likely to
>> be what happens in practice).
>
>
> I think that's a defect in the (non-normative) note in 5.3.4/17, which is
> assuming that this follows from the rule in 8.5/12.  But I don't think it
> does, because in this case the storage has been initialized and therefore is
> no longer indeterminate.
>
> So I think the placement new form is OK.

Huh?  Doesn't placement new end the lifetime of the object that resided
at the address (by re-using memory) and start lifetime of a new object
at that address?

So how can the new object see the storage contents of the old object?

GCC certainly doesn't support type-punning via placement new.

As with all methods you might be lucky with recent enough GCC
versions as if they see a must-alias they don't try to disambiguate
further with TBAA.

Richard.


> Jason
>