"Victor Bazarov" <v.Abazarov@comAcast.net> wrote in message
news:OJ0Fb.612029$Fm2.553208@attbi_s04...

"Jakob Bieling" <netsurf@gmy.net> wrote...

I have a question about casting and using the casted pointer:

Suppose

I

have a 'base' class and a 'derived' class (which is derived from

'base').

Now I have this scenario:

base* p1 = new base ();
base* p2 = new derived ();

derived* d1 = (derived*) p1;
derived* d2 = (derived*) p2;

Does the cast itself already produce undefined behaviour?

No.

Or will I get
there when using 'd1'?

Yes.

I am asking because I have a similiar scenario, where
the casted pointer is used in an if-statement, like this:

if (p2->is_derived () && d2->func_only_present_in_derived_class

{
}

If the function 'is_derived' doesn't attempt to interpret the pointer
_as__if_ it points to an object of class 'derived', but uses some other
mechanism (that actually works), you should be fine.

I can imagine that these classes should work:

class base {
public:
virtual ~base() {}
virtual bool is_derived() const { return false; }
};

class derived: public base {
public:
bool is_derived() const ( return true; }
bool func_only_present_in_derived_class();
};

This is pretty much exactly what my class design looks like *g*

Victor Bazarov schrieb:

"Jakob Bieling" <netsurf@gmy.net> wrote...

I have a question about casting and using the casted pointer: Suppose

I

have a 'base' class and a 'derived' class (which is derived from 'base').
Now I have this scenario:

base* p1 = new base ();
base* p2 = new derived ();

derived* d1 = (derived*) p1;
derived* d2 = (derived*) p2;

Does the cast itself already produce undefined behaviour?

No.

I think you might get in trouble in general, e.g. if you have

class base {
int x;
};
class base1{
int y[10000];
};
class derived :public base1,public base{};

In this case d1 really points to the middle of nowhere and this seem as illegal as

int x[7];
int * y=x-1;

any comments?
Klaus
[snip]