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;