qobjectcast

qobject_cast is a type-safe downcast utility in the Qt framework that uses the Qt meta-object system to cast pointers among QObject-derived types. It returns a pointer to the requested type if the object's runtime type matches the target type or a subclass, and returns nullptr otherwise. The cast relies on Qt’s runtime type information rather than C++ RTTI alone.

qobject_cast is intended for use with QObject hierarchies and with types that participate in the Qt meta-object system via the Q_OBJECT macro. The typical usage is qobject_cast<T*>(object), where object is a QObject* or null. The function checks the object's meta-object information to determine whether the cast is valid and returns a pointer of type T* or nullptr if the cast cannot be performed.

Compared with dynamic_cast, qobject_cast uses Qt’s meta-object data rather than C++ RTTI. This makes it suitable for Qt-specific scenarios, including downcasting within or across Qt-based modules, where only QObject-derived types with Q_OBJECT provide reliable runtime information. It is not applicable to non-QObject types or to classes without the Q_OBJECT macro.

Example:

QObject *obj = someObject;

QLabel *label = qobject_cast<QLabel*>(obj);

if (label) {

// use label

}

Limitations and notes:

- Both source and target types must derive from QObject and participate in the meta-object system (must have Q_OBJECT).

- It cannot be used for non-QObject types or for arbitrary C++ downcasts.

- It is most useful when working with QObject hierarchies in Qt-based codebases.