4 ms·

I understand the upcast (which is certainly legal but it forces the casting code to know the depth of the inheritance hierarchy - as in &derived->base1.base2),

• haberman 11y ago
  > I understand the upcast (which is certainly legal but it forces the casting code to know the depth of the inheritance hierarchy - as in &derived->base1.base2) If this is inconvenient, just casting directly to Base pointer is also legal. > but what's the argument making the downcast back to Derived legal C? The justification comes from this part of the C standard (C99 6.7.2.1 p13): Within a structure object, the non-bit-field members and the units in which bit-fields reside have addresses that increase in the order in which they are declared. A pointer to a structure object, suitably converted, points to its initial member (or if that member is a bit-field, then to the unit in which it resides), and vice versa. There may be unnamed padding within a structure object, but not at its beginning. It follows that: Derived *d = GetDerived(); // This is legal: a pointer to Derived, suitably converted, // points to its initial member "base": Base *base = (Base*)d; // This is also legal: a pointer to Derived.base, the initial // member of Derived, suitably converted, points to Derived. Derived *d2 = (Derived*)base;