Is converting a reinterpret_cast'd derived class pointer to base class pointer undefined behavior?

Question

Have a look at is simple example:

struct Base { /* some virtual functions here */ };
struct A: Base { /* members, overridden virtual functions */ };
struct B: Ba         


        

Answer 1

static_cast (or an implicit derived-to-base-pointer conversion, which does exactly the same thing) is substantially different from reinterpret_cast. There is no guarantee that that the base subobject starts at the same address as the complete object.

Most implementations place the first base subobject at the same address as the complete object, but of course even such implementations cannot place two different non-empty base subobjects at the same address. (An object with virtual functions is not empty). When the base subobject is not at the same address as the complete object, static_cast is not a no-op, it involves pointer adjustment.

There are implementations that never place even the first base subobject at the same address as the complete object. It is allowed to place the base subobject after all members of derived, for example. IIRC the Sun C++ compiler used to layout classes this way (don't know if it's still doing that). On such an implementation, this code is almost guaranteed to fail.

Similar code with B having more than one base will fail on many implementations. Example.