class Item

{

  virtual Item *get_copy(THD *thd)=0;

}

class Item_func

{

  Item *get_copy(THD *thd) override

  { return get_item_copy<Item_func>(thd, this); }

}

classs Item_func_descendant : public Item_func

{

  // get_copy() not defined, there's no complains from the compiler

  // once it's defined in Item_func

}

{

  Item* instance_of_item_func_descendant;

  Item* copy= instance_of_item_func_descendant->get_copy(thd);

  typeid(copy) == "Item_func" although "Item_func_descendant" expected!

}