c++,qt,inheritance,polymorphism , How do I properly organize my inheritance classes to take advantage of polymorphism?


How do I properly organize my inheritance classes to take advantage of polymorphism?

Question:

Tag: c++,qt,inheritance,polymorphism

I am trying to redesign my item class. I am having trouble picturing how things should work.

Current implementation:

class Item : public QGraphicsItem
{
public:
    typedef enum  { PolygonType = QGraphicsItem::UserType + 1 } ShapeType;
    Item() {...} 
    Item(const Item &copyItem) // copy constructor
    {   m_shapeType = copyItem.getItemShape();
        m_color = copyItem.getItemColor();
        setPos(copyItem.pos()); ...}
    QRectF boundingRect() const;
    QPainterPath shape() const;
    void setItemShape(ShapeType s) { m_shapeType = s; }
    ShapeType getItemShape() const { return m_shapeType; }
    int type() const { return m_shapeType; }   // this will replace the above to allow QGraphicsItem casts
    void setItemColor(QColor c)    { m_color = c; }
    QColor getItemColor() const    { return m_color; }
    ....
protected:
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option,
               QWidget *widget);
    QColor m_color;
    ShapeType m_shapeType;
    ....
};

QPainterPath Item::shape() const
{
    QPainterPath path;
    switch (m_shapeType)
    {
    case Item::PolygonType:
        ...
        break;
    ....
    }
}
QRectF Item::boundingRect() const
{
    return QRectF(...);
}
void Item::paint(QPainter *painter, const QStyleOptionGraphicsItem */*option*/, QWidget */*widget*/)
{
    switch (m_shapeType)
    {
    case Item::PolygonType:
            painter->drawPolygon(shape().toFillPolygon());
            break;
    ....
    }
}

Based on selections, I create an item and add to a canvas... then once all changes have been made, I add it to a QGraphicsScene.

Required tasks:

The processing of the scene contains iterations of the type

Item* item1 = new Item(*renderArea->item);
// make some changes on the item copy, then add it to a scene
collectionView->scene()->addItem(item1);

// in another function...
for(int i = 0; i < collectionView->scene()->items().size(); ++i)
{        
    Item* item = new Item(*(dynamic_cast<Item*>(
                           collectionView->scene()->items().at(i))));
   ...
}

What I am trying to achieve

This is a minimal approach, but I have to extend functionality to items that extend other classes, like QGraphicsPixmapItem (without having to add a pixmap to every item that does not need it) or QGraphicsSvgItem (again, without having to add so much overhead to items that don't need it).

So what I am trying:

class PolyItem : public Item
{
public:
    PolyItem(): Item()
    {
        m_shapeType = Item::PolygonType;
    }
    PolyItem(PolyItem& copy): Item::Item(copy)
    {
    }
    virtual void paint(QPainter* painter,
                       const QStyleOptionGraphicsItem* option,
                       QWidget* widget = NULL)
    {
        painter->drawPolygon(shape().toFillPolygon());
    }
};

or

class PixMapItem : public Item, public QGraphicsPixmapItem
{
public:
    PixMapItem(): Item()
    {
        m_shapeType = Item::PixmapType;
        setShapeMode(QGraphicsPixmapItem::BoundingRectShape);
    }
    PixMapItem(PixMapItem& copy): Item::Item(copy)
    {
        setPixmap(copy.pixmap());
        setShapeMode(QGraphicsPixmapItem::BoundingRectShape);
    }
    virtual void paint(QPainter* painter,
                       const QStyleOptionGraphicsItem* option,
                       QWidget* widget = NULL)
    {
        painter->drawPixmap(boundingRect(), pixmap());
    }
};

The question:

-----> I was thinking of making a different constructor that creates the correct shape, in class Item - but that seems to mean that Item requires class polygon that requires class Item... circular dependency ?

What I can't visualize:

collectionView->scene()->addItem(item1);

If my item is of polygon type, will the Item::paint() function - and all the other associated functions - be called, or the PolyItem::paint() ?
Do I have to insert switch-case code to determine and cast every single instance, of every single type ?


Answer:

Probably it worth to reduce your Item class to BaseItem containing everything you are adding to an Item yourself, and all functions you expect any Item to have (as pure virtual if they cannot be implemented without QGraphics*):

class BaseItem {
public:
    void setItemColor(QColor c)    { m_color = c; }
    QColor getItemColor() const    { return m_color; }
    ....
protected:
    QColor m_color;
    ShapeType m_shapeType;
};

Then

class Item : public BaseItem, public QGraphicsItem
class PixMapItem : public BaseItem, public QGraphicsPixmapItem

and so forth - avoiding circular dependencies. Any Item can be referred to as BaseItem, and provides common interface defined by BaseItem. Each child provides specialization of that interface to proper QGraphics*.

Problem with your original design is that item inherited from QGraphicsItem while it shouldn't, since you plan further specialize it for QGraphicsPixmapItem, which contains QGraphicsItem in itself already.

Edit

Now, you treat all items as BaseItem when you iterate through them - which means that BaseItem has to provide an interface all Items adhere to. Given examples you provided in your question, one way to do it:

class BaseItem {
  virtual void AddToScene(QGraphicsScene* scene) = 0;
  // Caller takes ownership.
  virtual BaseItem* CreateCopy() = 0;
};

Required tasks:

The processing of the scene contains iterations of the type

BaseItem* item1 = renderArea->item->CreateCopy();
// make some changes on the item copy, then add it to a scene
item1->AddToScene(collectionView->scene());

Each derived class implements interface defined by BaseItem, while you treat all Items as BaseItem in your generic operations. That also makes code easily extensible - adding new Item type only requires implementing derived class, but existing code stays as is - so BaseItem needs to be carefully designed to provide interface for operations common for all items.

class Item : public BaseItem, public QGraphicsItem {
      void AddToScene(QGraphicsScene* scene) override { scene->addItem(this); }
      // Caller takes ownership.
      BaseItem* CreateCopy() override { return new Item; }
};

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