Josh, what's your opinion about this situation?
You can't downcast to a derived type if the object isn't actually an instance of the derived type. That seems straightforward, no?
The reason your example is illegal is that you are casting to pointer-to-"struct derived", but the thing being pointed to is not actually a "struct derived."
The "physical subtyping" pattern works because the C standard says that a pointer to a struct, suitably converted, also points to its first member. So a pointer-to-Derived, converted to a pointer-to-Base, points at Derived's first member. But a pointer-to-Base doesn't point at a Derived unless that object actually is a Derived. So the downcast is only legal if the object actually is a Derived.
But if the object isn't actually a Derived, you can't cast to pointer-to-Derived:
Derived derived;
Derived *pDerived = &derived;
// This is legal because it's equivalent to:
// Base *pb = &derived.base;
//
// ie. there actually is a Base object there that the
// pointer is pointing to.
Base *pBase = (Base*)pDerived;
// This is legal because pBase points to the initial member
// of a Derived. So, suitably converted, it points at the
// Derived.
//
// The key point is that there actually is a Derived object
// there that we are pointing at.
pDerived = (Derived*)pBase;
Base base;
// This is illegal, because this base object is not actually
// a part of a larger Derived object, it's just a Base.
// So we have a pDerived that doesn't actually point at a
// Derived object -- this is illegal.
pDerived = (Derived*)&base;
// Imagine if the above were actually legal -- this would
// reference unallocated memory!
pDerived->some_derived_member = 5;All the compilers I have used will cheerfully reference unallocated memory; I thought the behavior was undefined.
After spending too much of my life chasing these bugs, here the compiler will do exactly what you told it to, which probably means making your day miserable.
Let me try to think of a good way to update the post to capture this better...
You can work around this by clicking on the link for the post itself (ie. "3 minutes ago") which allows you to reply immediately.
It also violates string aliasing because 6.5.7. only works for one way. See my other comment.
Derived can alias Base, but not vice-versa.