Re: Foward decleration casting craziness
speedplane wrote:
Hi there...
I just found a bug in a very well known compiler and I was wondering if
others have found the same problem. Consider the code snippet
class C; // Foward decleration
class A {
public:
// Here we cast to C but we don't know how to cast an A to
C* getAasC{ return (C*)this; };C
int a;
};
Note that this code will compile (in my compiler at least) even though
it does not know how to cast an A to a C. In another file say we define
C:
class B {
public:
int b;
};
class C : public B, A {
public;
int c;
}
In the first code snippet, the compiler does not know how to cast an A
to a C. Therefore it does a reinterpret cast instead of a static cast.
Consider the following code:
A * a = new C();
a->a = 50;
printf("A: %d, C: %d", a->a, a->getAasC()->c)
Depending on how you compiler does the reintpret cast the above will
either segfault or print:
A: 50, C: 50
My compiler didn't even give me a warning! I think it should give you
an error if you try to cast something and it doesn't know how to do the
cast.
Has anyone else run into something like this?
This is not a bug. You are not casting to a "C" but rather to a
"pointer to a C." And the C-style cast used instructs the compiler to
use any casting method (other than dynamic).
In this case it had no choice but to use reinterpret_cast, and hope
that the programmer knows what he is doing.
In other words, it did exactly what you told it to do.
If you want help from the compiler, you will have to tell it what you
are trying to do. In this case, you would write
C* getAasC{ return static_cast<C*>(this); } and if the compiler does
not howl THAT is a compiler bug.
I leave it as a excercise for the reader on how to implement getAasC
correctly, but virtual C* getAasC()=0; comes to mind.....
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]