Promoting a derived struct to another deriving struct

[luckiejacky]

struct A
{
};

struct B : public A
{
};

A* a = new A();
B* b = (B*) a;

When will this downcast fail? I just got a bad pointer out of this cast...(0xbaadf00dbaadf00d )
Thanks
Jack

[VictorN]

struct A
{
   int a;
};

struct B : public A
{
   double b;
};

A* a = new A();

The created *a has no place for double b; that is appeared to belong in struct B.

[luckiejacky]

Why this thing is always a problem? I mean I create FRAME's inside this function (the B) and return to the caller, then the caller accepts it as a derived class (the A)

newFrame->pFrameSibling 
	= (FRAME*)CopyFrameHierarchy((const FRAME*)other->pFrameSibling);

Even on an explict cast

[VictorN]

Why this thing is always a problem? I mean I create FRAME's inside this function (the B) and return to the caller, then the caller accepts it as a derived class (the A)

But in your OP the B is a derived class while A is the base one!