[Bug c++/22132] New: Wrong code: upcasting a const class pointer to struct the class derives from

["scott dot tupaj at line6 dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Upcasting a const class pointer for a class that was derived from a struct causes the upcasted pointer to 
incorrectly access members of the derived struct.

Conditions:
1) The derived class object must have a virtual destructor (or possibly any v-table entries)
2) The upcast must be a straight 'C' cast that does not preserve the const.

I've attached a code sample that demonstrates the error, but this is the scenario:

struct foo {
   int a;
   int b;
};

class Foobar : public foo {
public:
    Foobar() { a = 1; b = 2; };
    virtual ~Foobar() {};
};

Foobar obj;
const Foobar* objPtr = &obj;
foo* f = (foo*)objPtr;

contents of f->a and f->b during runtime are not 1 and 2 as expected.
f->a is garbage
f->b is 1 (expected value of f->a)

-- 
           Summary: Wrong code: upcasting a const class pointer to struct
                    the class derives from
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: scott dot tupaj at line6 dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22132