Understanding QGraphicsPixmapItem Memory Management in Qt Widgets

1. Memory Management
   When a QGraphicsPixmapItem is destroyed, the destructor of its parent class (QGraphicsItem) is likely responsible for releasing any memory associated with the item itself. This might involve deallocating the internal data structures used to represent the pixmap.

2. Scene Removal
   If the QGraphicsPixmapItem was added to a QGraphicsScene, the destructor might also be responsible for removing the item from the scene. This ensures the item is no longer drawn or managed by the scene.

What you can do instead

While you cannot directly call the destructor, you can control the lifetime of a QGraphicsPixmapItem by:

• Ownership by the Scene
  When you add a QGraphicsPixmapItem to a QGraphicsScene, the scene takes ownership of the item. The scene will automatically manage the item's lifetime and call the destructor when the scene itself is destroyed.

• Manual Deletion
  If you create the QGraphicsPixmapItem with new, you can explicitly delete it using delete when you're done with it. This will trigger the destructor and any associated cleanup.

---

Example 1: Manual Deletion

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QPixmap>

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

  // Create a scene and view
  QGraphicsScene scene;
  QGraphicsView view(&scene);

  // Load a pixmap
  QPixmap pixmap("image.png");

  // Create a QGraphicsPixmapItem manually
  QGraphicsPixmapItem* item = new QGraphicsPixmapItem(pixmap);

  // Add the item to the scene
  scene.addItem(item);

  // Set the item position (optional)
  item->setPos(100, 50);

  // Show the view
  view.show();

  // ... (Your application logic here)

  // Manually delete the item when done
  delete item;

  return app.exec();
}

In this example, we create a QGraphicsPixmapItem using new and then explicitly delete it using delete when we're done with it. This ensures the memory used by the item is properly released.

Example 2: Ownership by the Scene

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QPixmap>

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

  // Create a scene and view
  QGraphicsScene scene;
  QGraphicsView view(&scene);

  // Load a pixmap
  QPixmap pixmap("image.png");

  // Create a QGraphicsPixmapItem directly on the scene (no new)
  QGraphicsPixmapItem* item = scene.addPixmap(pixmap);

  // Set the item position (optional)
  item->setPos(100, 50);

  // Show the view
  view.show();

  // ... (Your application logic here)

  // Scene manages the item lifetime

  return app.exec();
}

Here, we create the QGraphicsPixmapItem directly on the scene using addPixmap. Since it's added to the scene, the scene takes ownership of the item and will automatically call the destructor (inherited from QGraphicsItem) when the scene itself is destroyed.

---

1. • This approach is suitable when you have fine-grained control over the item's lifetime.
   • You create the item with new and then explicitly delete it with delete when you're finished using it.

2. Ownership by QGraphicsScene
   

   • This is the simpler approach and recommended for most cases.
   • You add the item directly to a QGraphicsScene using methods like addPixmap or addItem.
   • The scene takes ownership of the item and will automatically manage its lifetime, including calling the destructor when the scene itself is destroyed.

3. Smart Pointers
   

   • While not strictly necessary due to Qt's automatic memory management, you can use smart pointers like std::unique_ptr or std::shared_ptr for improved code clarity and exception safety.
   • These pointers manage the lifetime of the QGraphicsPixmapItem object and ensure proper deletion even in case of exceptions.

Approach	Description	Advantages	Disadvantages
Manual Deletion (delete)	You create and delete the item manually.	Fine-grained control over lifetime	Requires manual memory management, can lead to memory leaks if not done properly.
Ownership by QGraphicsScene	Add the item to a scene, scene manages lifetime.	Simple and recommended, automatic deletion.	Less control over lifetime compared to manual deletion.
Smart Pointers (std::unique_ptr, std::shared_ptr)	Use smart pointers to manage object lifetime.	Improved code clarity, exception safety.	Can introduce additional complexity compared to simpler approaches.