Undo redo with automatic deletion mechanism

My application is being developed in C ++ using Qt and uses signals and slots.

Let's say I have the following classes (pseudo C ++ code):

class Ball
{
    Color m_Color;
    int m_Size;
};

class Player
{
public:
    setBall(Ball* pBall)
    {
        if (pBall != m_pBall)
        {
            Ball* pPreviousBall = m_pBall;
            m_pBall = pBall;
            emit notifyBallNotUsed(pPreviousBall);
        }
    }

    Ball* getBall();

signals:
    void notifyBallNotUsed(Ball*);

private:
    String m_Name;
    Ball* m_pBall;
};

class GeneralHandler
{
public:
    addBall(Ball* pBall);
    deleteBall(Ball* pBall);


    addPlayer(Player* pPlayer)
    {
        connect(pPlayer, SIGNAL(notifyBallNotUsed(Ball*)), this, SLOT(onBallUsageChanged(Ball*)));
        ...
    }
    deletePlayer(Player* pPlayer);
    {
        disconnect(pPlayer, SIGNAL(notifyBallNotUsed(Ball*)), this, SLOT(onBallUsageChanged(Ball*)));

        onBallUsageChanged(pPlayer->getBall());
        ....
    }

private slots:
    void onBallUsageChanged(Ball* pBall)
    {
        if (isNotUsedAnymore(pBall))
        {
            m_BallList.remove(pBall);
            delete pBall;
        }
    }

private:
    bool isNotUsedAnymore(Ball* pBall); // Check if the given ball is still used by at least one player

    List<Player*> m_PlayerList;
    List<Ball*> m_BallList;
};

      

        
        
        
      

    

In my application, the user can add / remove a player, and for each player, select the color and size of the ball. Behind the hood, the GeneralHandler is responsible for storing the balls and removing them. It is possible that two players are using the same ball.

When a player is removed if the ball is no longer in use, the GeneralHandler must remove it (or keep it if the ball is still in use by another player). If the ball that the player is using changes, the previous ball, if no longer in use, must also be removed by General Handler.

So far so good.

Now I want to add undo / redo functionality to my application using command pattern and this is where I am stuck. Let's say I have something like this:

class ChangePlayerBall : public QUndoCommand
{
public:
    ChangePlayerBall(Player* pPlayer, Ball* pNewBall)
    {
        m_pPlayer = pPlayer;
    }

    void redo();
    void undo();

private:
    Player* m_pPlayer;
};

      

        
        
        
      

    

I think the redo () method will look like this:

void ChangePlayerBall::redo()
{
    m_pPlayer->setBall(pNewBall);
}

      

        
        
        
      

    

If nothing has changed in the code above, the previous ball will be removed if it is no longer used by other players. This will be a problem when implementing the undo () method: if the previous ball was deleted, I don't know what its characteristics were, and the undo command won't be able to recreate it. Or maybe I should keep the previous ball, but how does the undo / redo command know if this previous ball still exists or has been removed by the handler? Or maybe this mechanism for removing the ball, once it is no longer used, should be implemented in the undo command? The problem is that the undo command will have many dependencies on many other classes. Another problem is that this code will be partially duplicated in the DeletePlayer command, which should do something like this:

class DeletePlayer : public QUndoCommand
{
public:
    DeletePlayer(Player* pPlayer);

    void redo();
    void undo();
...
};

      

        
        
        
      

    

Hope my explanation is clear!

How would you solve this problem? I cannot find a satisfactory solution.

Thank!