How to properly override virtual method to add functionality? - c++

Question

In base class I have simple remove by id virtual function, however in derived class I need also emit a signal (notify) after removing.
In base class. Below is the default implementation of function
void Ui::GameEntityList::remove_games_by_sport_id_virt(const QString &sport_id)
{
for(QList<GameEntity*>::iterator it = m_game_list.begin();
it != m_game_list.end();)
{
GameEntity* tmp = (*it);
if(tmp->get_sport_id() == sport_id)
{
it = m_game_list.erase(it);
delete tmp;
tmp = 0;
}
else
{
++it;
}
}
}
In derived class. Overriding
void Ui::GameWidgetList::remove_games_by_sport_id_virt(const QString &id)
{
QList<GameEntity*>::iterator it;
for(it = m_game_list.begin(); it != m_game_list.end();)
{
GameWidget* tmp = dynamic_cast<GameWidget*>(*it);
Q_ASSERT(tmp != NULL);
if(tmp->get_sport_id() == id)
{
it = m_game_list.erase(it);
emit remove_game_in_monitor(tmp->get_id(), this->get_monitor_number()); // need to emit this signal
delete tmp;
tmp = 0;
}
else
{
++it;
}
}
this->set_number_of_games(m_game_list.size()); // need to call this function
}
I can't manage a way to avoid code duplication. Should I have an empty virtual notify() function and call it after removing an element? That way I can override notify() in derived to do the job. Is that an acceptable decision? Is implementing remove by id in base class unnecessary?

Answer 1

In your case, it is not that bad regarding amount of duplicated code. But, anyway, in situations like this, you may want to make this function non-virtual and move this customizable functionality to other virtual functions:
class GameEntityList
{
private:
virtual void on_erase(GameWidget* w)
{
//empty
}
virtual void on_finish(GameWidget* w)
{
//empty
}
//others
};
class GameWidgetList : public GameEntityList
{
private:
virtual void on_erase(GameWidget* w)
{
remove_game_in_monitor(w->get_id(), this->get_monitor_number());
}
virtual void on_finish(GameWidget* w)
{
this->set_number_of_games(m_game_list.size());
}
//others
};
And then:
void Ui::GameEntityList::remove_games_by_sport_id_virt(const QString &id)
{
QList<GameEntity*>::iterator it;
for(it = m_game_list.begin(); it != m_game_list.end();)
{
if(tmp->get_sport_id() == id)
{
it = m_game_list.erase(it);
this->on_erase(tmp); //customizable
delete tmp;
tmp = 0;
}
else
{
++it;
}
}
this->on_finish(); //customizable
}