On 07/01/2011 18:34, Alf P. Steinbach /Usenet wrote:
* LR, on 07.01.2011 06:59:
Alf P. Steinbach /Usenet wrote:
It so happened that this vagueness (allowing non-contiguous regions)
was done deliberately in order to support multiple virtual
inheritance, but that at least some committee members saw it as not
only necessary for virtual inheritance, but also allowing
implementations that scatter parts of objects all around in memory.
Thus, the wording in the standard does not say right out that it's in
support of virtual inheritance. The standard intentionally leaves
the door open for what we'd now regard as "perverse" implementations.
:-)
Would I be correct in inferring that non-contiguous classes are no
longer regarded as necessary? If not, does the current draft still
support a requirement for this?
Did any implementation create objects with non-contiguous memory? Anyone
know?
However, in thread "Re: technical correctness"
in Message-ID:<AYqdnW1Lpptx9bnQnZ2dnUVZ8nydnZ2d@bt.com>
Francis Glassborow<francis.glassborow@btinternet.com>
wrote: "All subobjects must reside in a single contiguous region which
is the object. Otherwise sizeof and operator new would be useless:"
Would this also be a requirement for the offsetof macro as well? I
don't recall if offsetof is in the standard, but it is in n3126.
If I understand what they've written correctly, I think that either Mr.
Steinbach or Mr. Glassborow is wrong. Perhaps someone could clarify?
Provide a cite from either the standard or the latest draft?
Well, Francis' comment about the non-usefulness of 'sizeof' would apply
also to an object with a virtual base.
struct Base{ int x; };
struct D1: virtual Base { int d1; };
struct D2: virtual Base { int d2; };
struct Bottom: D1, D2 {};
Bottom doh;
Within the object 'doh' you can have the D1 sub-object of size
sizeof(D1), but then, since a part of it is shared with the D2
sub-object, the D2 sub-object can't be of size sizeof(D2). And vice
versa. But you can still pick up a reference to, say, the D2 sub-object,
and pass it to some function that doesn't know about Bottom or anything
other than that it is a D2 object, so on the inside of that object there
is necessarily some indirection referring to the Base sub-object: the
Base sub-object is not necessarily directly contained.
So Francis' /argument/ is lacking, but in practice his conclusion is
correct when "the object" is replaced with "the object of the most
derived class".
The purely formal and purely academic problem is that the standard
doesn't guarantee even that... The C++98 standard allows the same
shenanigans that the compiler must insert for D1 and D2 objects, for any
non-POD object. But, since I don't know of any actual implementation
where it doesn't hold, when I try to explain this to novices I just say
that a /variable/ is a contiguous region of storage, because it is in
practice.
Cheers & hth.,
- Alf
sub=objects. The latter can be disjoint and the former cannot be. You do
not even have to go as far as using a virtual base. A derived class can