Assumptions about the memory location of objects

Question

For which types of types (standard layout, POD, trivial type, alignment restrictions ...) are the following assumptions allowed?

Bis a subclass Aand B* bpointer to an object B.

• A* a = bhas the same address as B, i.e.

  static_cast<A*>(b) == reinterpret_cast<A*>(b)
  

• B bs[n]is an array of objects B.

  &bs[i] == static_cast<B*>(reinterpret_cast<unsigned char*>(bs) + i * sizeof(B))
  

• offsetofcan be used to access data elements Aand B, from B:

  int i = *static_cast<int*>(
      reinterpret_cast<unsigned char*>(b) + offsetof(B, m_i));
  

EDIT: changed void*to unsigned char*for pointer arithmetic (with 1 byte unit)

Answer 1

A B B B, .

offsetof .

B, , .