void GenCollectedHeap::object_iterate_since_last_GC(ObjectClosure* cl) {

  {- -------------------------------------------
  (1) GenCollectedHeap 内の各 Generaiton オブジェクトに対して, 
      Generation::object_iterate_since_last_GC() を呼び出す.
      ---------------------------------------- -}

      for (int i = 0; i < _n_gens; i++) {
        _gens[i]->object_iterate_since_last_GC(cl);
      }
    }