Component Objects: C Nested Structures

C interface implementations comprise data structures nested within the object’s data structure. Each interface structure contains a VTBL pointer as its first member (pVtbl), a pointer to the object (pCObj), and a count of the external references to the interface (m_RefCount). The order of the members in the interface structures is identical, to facilitate code sharing.

typedef struct CObj { 
  ULONG            m_ObjRefCount; 
  LPSTORAGE        m_pStg; 
  LPOLEOBJECT      m_pOleObj; 
  struct CDOC    *  m_pCDoc; 
 
  struct InterfaceA  { 
    LPVTBL        pVtbl; 
    struct CObj  *  pCObj; 
    ULONG        m_RefCount; 
  } m_InterfaceA; 
 
  struct InterfaceB { 
    LPVTBL       pVtbl; 
    struct Obj  *    pCObj; 
    ULONG        m_RefCount; 
  } m_InterfaceB; 
 
} COBJ;