On Mon, Jan 6, 2014 at 3:44 PM, Richard Smith <richardsmith@google.com> wrote:
On Mon, Jan 6, 2014 at 10:22 AM, Jason Merrill <jason@redhat.com> wrote:
On 01/06/2014 04:26 AM, Fabio Fracassi wrote:
> if it is not (legal): could we make it legal or would we run afoul of
> the aliasing rules?

The access is not allowed by the aliasing rules in 3.10.  But it seems
that this would be:

struct B {
   int i;
};

struct D {
   B bmem;
   void foo() { /* access bmem.i */ }
};

B b;
reinterpret_cast<D&>(b).foo();

because B is a non-static data member of D, and 9.2/19 guarantees that
the address of D::bmem is the same as the address of the D object.

How is that fundamentally different? 9.3.1/2 makes that UB too, if 'reinterpret_cast<D&>(b)' does not refer to an object of type 'b'.

... an object of type 'D'. Sorry!
 
And within D::foo, the implicit this->bmem would have the same problem.


If I might play devil's advocate for a moment...

  struct B { int i; };
  struct D : B {
    void foo();
  };

  B b;

I claim this line starts the lifetime of an object of type D. Per [basic.life]p1, the lifetime of an object of type 'D' begins when storage with the proper alignment and size for type T is obtained (which "B b" happens to satisfy). The object does not have non-trivial initialization, so the second bullet does not apply.

(This is the same argument that makes this valid:

  D *p = (D*)malloc(sizeof(D));
  p->foo();

... so any counterargument will need to explain why the two cases are fundamentally different.)

Then:

  reinterpret_cast<D&>(b).foo();

... is valid, because the cast produces the same memory address, and that memory address contains an object of type 'D' (as claimed above).