Re: Foward decleration casting craziness

From:
=?windows-1252?Q?Martin_Vejn=E1r?= <avakar@volny.cz>
Newsgroups:
comp.lang.c++.moderated
Date:
17 Jul 2006 16:02:58 -0400
Message-ID:
<e9fo1f$1p1c$1@ns.felk.cvut.cz>
speedplane wrote:

Hi there...


I've read through other responses, and it seems to me they've missed the
point. The following is the code in question (with few corrections and
modifications).

struct C;

struct A
{
    C * getAasC()
    {
        return (C*)this;
    };

    int a;
};

struct B
{
    int b;
};

struct C : public B, public A
{
    int c;
};

int main()
{
    A * a = new C();
    a->getAasC()->c = 50;
}


The relevant paragraph [5.4/6] states:

The operand of a cast using the cast notation can be an rvalue of
type ?pointer to incomplete class type?. The destination type of a
cast using the cast notation can be ?pointer to incomplete class
type?. In such cases, even if there is a inheritance relationship
between the source and destination classes, whether the static_cast
or reinterpret_cast interpretation is used is unspecified.


Whether the above program is a subject to Undefined Behavior is
unspecified, but the program is well-formed and should compile. On
MSVC8, reinterpret_cast is done, resulting in possible memory
corruption. That is, however, not a compiler bug. I agree that a warning
can be issued, that's, however, beyond Standard's control.

--
Martin

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