Re: Structure mapping using reinterpret_cast.

"Alf P. Steinbach" <>
Wed, 24 Jun 2009 13:28:37 CST
* Bart van Ingen Schenau:

Alf P. Steinbach wrote:

* Olivier:

That is, given :

struct A { int a; int b; };
struct B { int c; };

the following code is guaranteed to work :

     A a;
     B &b = *reinterpret_cast<B *>(&a);

     b.c = 12;
     assert(a.a == b.c);

And this assert should never fail.

The above is formally guaranteed by ?9.2/17 and flatly contradicted by
?5.2.10 (in particular ?5.2.10/1 "No other conversion can be performed

That is both false.

I'm sorry, that's incorrect.

Clause 9.2/17 only guarantees the following:

   A a;
   a.a = 0;
   int* p = reinterpret_cast<int*>(&a);
   *p = 42;
   assert(a.a == 42);

You apparently failed to consider that you can in turn, by ?9.2/17, cast your p
further to B*, and have a valid pointer to B*.

Or if you considered it and thought that that couldn't be allowed the rules,
then to effect different behavior you would have to postulate phat pointer magic
where a pointer dynamically retains knowledge of at least one of the types from
which it has been derived just to foul up common usage, which for a general
mechanism means dynamically retaining an unbounded list of original types from
which the pointer has been derived by successive well-defined ?9.2/17 casts...

There is no such foul-it-up magic (even though phat pointers possibly do exist).

Note, in that connection, that pointers to class type objects all have the same
representation, since you can have a pointer to incomplete type.

So, in the local context of not considering the standard's contradiction
elsewhere, and discounting magic, silly word-play and so on, A* -> B* cannot
yield a result different from A* -> int* -> B* which is well-defined.

Furthermore, the argument of contradiction in the standard does not rely on A*
-> B* being well-defined, but only on A* -> int* being well-defined.

So even one who does not "agree" with the well-definedness of A* -> B* :-),
cannot escape the conclusion of a defect, a self-contradiction, in the standard.

And clause 5.2.10 does not prohibit any pointer conversions (the
conversions A* -> B* and A* -> int* are both covered by 5.2.10/7).

?5.2.10 does not prohibit the conversion, right, and I shouldn't have pointed
parenthetically to that without explanation (easy to misunderstand), mea culpa
for the misleading parenthetical remark, but then I'm not perfect. :-)

?5.2.10/7 says "The result of such a pointer conversion is unspecified". This is
all that's allowed. Because there's no other discussion of the A* -> B* case in
?5.2.10, and as I quoted, "No other conversion can be performed explicitly".

Directly contradicting ?5.2.10/7, ?9.2/17 says "points to its initial member ...
and vice versa", which is not unspecified but yields a pointer you can use.

I'm aware that given an initial misunderstanding of what I wrote, the above may
sound as if I've finally found something to support my conclusion, sort of
weaseling out...

But note that I've discussed this at length and in detail in earlier clc++m
threads; again I apologize for this time not making the details utterly clear.

<snip - false conclusions>

I'm sorry, that's incorrect; see above.


Cheers & hth.,

- Alf

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