(ref.doc)subobjects

C++ Report
March/April 93 - Vol 5/No 3

p 51
Subobject members, by Stephen Dewhurst
- "natural" and "synthetic" inheritance relationships
- subobject member: a member that reroutes messages to its containing
  object in the same way that subobjects (base classes) allow
  customization of their response to messages by their complete object
  (most derived class)
- #define C_addr(C, M) ((C*)(((char*)this)-offsetof(C, M)))
  If code size, compile time, and build complexity are not important,
  a template function may be used in place of the macro.
- class Engine {
    // ...
    public:
      void start();
      void stop();
    private:
      EngineCustom _env;
      friend class EngineCustom;
  };
  Here we have defined a member for customization, a "hook", to be
  defined by the environment in which Engine is compiled.