Demystifying QGraphicsPolygonItem::anonymous (enum) and its Alternatives

Understanding QGraphicsPolygonItem

It allows you to create and manipulate polygons within your graphical user interface (GUI).
In Qt's graphics view framework, QGraphicsPolygonItem is a class that represents a polygon shape on the scene.

The anonymous (enum) Mystery

However, this is not an actual named enumeration (enum) within the class.
In the Qt documentation for QGraphicsPolygonItem, you might encounter anonymous (enum) listed among its members.

What it Really Means

This value is typically not an explicitly named enumeration type.
The anonymous (enum) is a placeholder used by the documentation to indicate that the type() function of QGraphicsPolygonItem returns an integer value that identifies the item's type.

Using type()

In the case of QGraphicsPolygonItem, type() returns an integer constant that corresponds to the QGraphicsItem::Type enumeration (usually the value QGraphicsItem::Type::Polygon).
It's used to determine the specific type of a graphics item in the scene hierarchy.
The type() function is a virtual function inherited from the base class QGraphicsItem.

Key Points

You don't need to directly interact with this unnamed enum.
It signifies an integer value returned by type() that identifies the item's type.
anonymous (enum) is not a real named enum within QGraphicsPolygonItem.

#include <QApplication>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QGraphicsPolygonItem>

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

  // Create a graphics scene
  QGraphicsScene scene;

  // Create a polygon with three vertices
  QPolygon polygon;
  polygon << QPoint(50, 50);
  polygon << QPoint(100, 100);
  polygon << QPoint(20, 100);

  // Create a QGraphicsPolygonItem from the polygon
  QGraphicsPolygonItem *polygonItem = new QGraphicsPolygonItem(polygon);

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

  // Create a graphics view to display the scene
  QGraphicsView view(&scene);
  view.show();

  // Get the item's type using type()
  int itemType = polygonItem->type();

  // Check if the type is Polygon (usually the value of QGraphicsItem::Type::Polygon)
  if (itemType == QGraphicsItem::Type::Polygon) {
    qDebug() << "Item is a polygon";
  } else {
    qDebug() << "Item is not a polygon";  // This shouldn't happen in this case
  }

  return app.exec();
}

In this code:

We create a QGraphicsScene object to hold our graphics items.
We define a polygon with three vertices.
We create a QGraphicsPolygonItem from the polygon.
We add the polygon item to the scene.
We create a QGraphicsView to display the scene in a window.
We call type() on the polygonItem to get its type as an integer.
We check if the type corresponds to QGraphicsItem::Type::Polygon (usually the case).

Checking Item Type with type()

If you simply want to verify that an item in your scene is a QGraphicsPolygonItem, you can use the type() function as shown in the previous example code. This is the most common approach.

int itemType = myGraphicsItem->type();
if (itemType == QGraphicsItem::Type::Polygon) {
  // Handle the polygon item
}

Dynamic Casting (if necessary)

In some rare cases, you might need to perform an operation specific to QGraphicsPolygonItem. If you're unsure of the item's exact type at compile time, you can use dynamic casting (C++ only) to check and cast the item if it's a polygon:

QGraphicsPolygonItem* polygonItem = dynamic_cast<QGraphicsPolygonItem*>(myGraphicsItem);
if (polygonItem) {
  // Perform operations specific to QGraphicsPolygonItem
  polygonItem->setBrush(Qt::red); // Example: Set the brush color
}

Using a Visitor Pattern (advanced)

For complex scenarios where you need to handle different item types in a generic way, you could consider the visitor pattern. This involves creating a visitor class with methods for handling each item type (polygon, rectangle, etc.). However, this is an advanced design pattern and is usually overkill for most cases.

The visitor pattern is an advanced approach for handling various item types in a generic way.
Dynamic casting might be necessary in rare cases where you need to perform polygon-specific operations and are unsure of the item type at compile time.
For most situations, using type() to check the item type is sufficient.