node_copy error, "QGraphicsRectItem is private"

[Ishmael]

I have two classes: "numberItem" and "numberLine".

The first is derived from QGraphicsRectItem. It works just fine by itself.

The other one is not really a class - more of a C-style struct - in the sense that all I would like to do is use it as a container for a bunch of related information. Among other things, I want it to contain an array of pointers to the first class.

Below is a simplified version of my code. Can anyone tell me what I'm doing wrong? Thanks so much!

First class:

class numberItem : public QGraphicsRectItem
{
public:
    numberItem(int cell_width);
private:
    int cell_width;
};
 
// Constructor for numberItem
numberItem::numberItem(int w = 5)
        : cell_width(w)
{
}

class numberItem : public QGraphicsRectItem
{
public:
    numberItem(int cell_width);
private:
    int cell_width;
};

// Constructor for numberItem
numberItem::numberItem(int w = 5)
        : cell_width(w)
{
}

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Second class:

class numberLine
{
public:
    // IF I COMMENT OUT THIS LINE, IT COMPILES JUST FINE
    QList<numberItem> all_number_items;
 
    QList<int> all_x;
    int num;
};

class numberLine
{
public:
    // IF I COMMENT OUT THIS LINE, IT COMPILES JUST FINE
    QList<numberItem> all_number_items;

    QList<int> all_x;
    int num;
};

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An the rest of the code:

numberLine get_num_line();
 
int main(int argc, char **argv){
  QApplication app(argc, argv);
 
  numberLine num_line;
  num_line = get_num_line();
 
  return app.exec();
}
 
numberLine get_num_line()
{
    numberLine x;
    return x;
}

numberLine get_num_line();

int main(int argc, char **argv){
  QApplication app(argc, argv);

  numberLine num_line;
  num_line = get_num_line();

  return app.exec();
}

numberLine get_num_line()
{
    numberLine x;
    return x;
}

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When I try to compile, I get a whole litany of unintelligible errors:

instantiated from `void QList<T>::node_copy(QList<T>::Node*, QList<T>::Node*, QList<T>::Node*) [with T = numberItem]'
instantiated from `void QList<T>::detach_helper() [with T = numberItem]'
instantiated from `QList<T>::QList(const QList<T>&) [with T = numberItem]'
instantiated from here
error: `QGraphicsRectItem::QGraphicsRectItem(const QGraphicsRectItem&)' is private

[scascio]

By default, you are not allowed to copy QGraphicsItem by default, except if you do so in derived class. The copy constructor is private as the compiler says.

In :

QList<numberItem> all_number_items;

QList<numberItem> all_number_items;

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The copy constructor is implicitly called by your container and that's why it compiles when you comment it.

The solution is to use a container of pointers that manages your derived items :

QList<numberItem*> all_number_items;

QList<numberItem*> all_number_items;

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or to change you design and instead of deriving QGraphicsRectItem, use a member:

class numberItem {
  public:
    numberItem(int w) : cell_width(w), item(0){}
    void setItem(QGraphicsRectItem* item);
    QGraphicsRectItem* getItem();
private:
    int cell_width;
    QGraphicsRectItem * item;
};

class numberItem {
  public:
    numberItem(int w) : cell_width(w), item(0){}
    void setItem(QGraphicsRectItem* item);
    QGraphicsRectItem* getItem();
private:
    int cell_width;
    QGraphicsRectItem * item;
};

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Remember that items added in the scene are owned by the QGraphicsScene, that will delete them when removed.

[Ishmael]

Very helpful. Thanks a lot!