QMetaType Class

 int id = QMetaType::type("MyClass");
 if (id != QMetaType::UnknownType) {
     void *myClassPtr = QMetaType::create(id);
     ...
     QMetaType::destroy(id, myClassPtr);
     myClassPtr = 0;
 }

Macro Documentation

Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE(Container)

This macro makes the container Container known to QMetaType as an associative container. This makes it possible to put an instance of Container<T, U> into a QVariant, if T and U are themselves known to QMetaType.

Note that all of the Qt associative containers already have built-in support, and it is not necessary to use this macro with them. The std::map container also has built-in support.

This example shows a typical use of Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE():

 #include <unordered_list>

 Q_DECLARE_ASSOCIATIVE_CONTAINER_METATYPE(std::unordered_map)

 void someFunc()
 {
     std::unordered_map<int, bool> container;
     QVariant var = QVariant::fromValue(container);
     // ...
 }

Q_DECLARE_METATYPE(Type)

This macro makes the type Type known to QMetaType as long as it provides a public default constructor, a public copy constructor and a public destructor. It is needed to use the type Type as a custom type in QVariant.

This macro requires that Type is a fully defined type at the point where it is used. For pointer types, it also requires that the pointed to type is fully defined. Use in conjunction with Q_DECLARE_OPAQUE_POINTER() to register pointers to forward declared types.

Ideally, this macro should be placed below the declaration of the class or struct. If that is not possible, it can be put in a private header file which has to be included every time that type is used in a QVariant.

Adding a Q_DECLARE_METATYPE() makes the type known to all template based functions, including QVariant. Note that if you intend to use the type in queued signal and slot connections or in QObject's property system, you also have to call qRegisterMetaType() since the names are resolved at runtime.

This example shows a typical use case of Q_DECLARE_METATYPE():

 struct MyStruct
 {
     int i;
     ...
 };

 Q_DECLARE_METATYPE(MyStruct)

If MyStruct is in a namespace, the Q_DECLARE_METATYPE() macro has to be outside the namespace:

 namespace MyNamespace
 {
     ...
 }

 Q_DECLARE_METATYPE(MyNamespace::MyStruct)

Since MyStruct is now known to QMetaType, it can be used in QVariant:

 MyStruct s;
 QVariant var;
 var.setValue(s); // copy s into the variant

 ...

 // retrieve the value
 MyStruct s2 = var.value<MyStruct>();

Some types are registered automatically and do not need this macro:

• Pointers to classes derived from QObject
• QList<T>, QVector<T>, QQueue<T>, QStack<T>, QSet<T> or QLinkedList<T> where T is a registered meta type
• QHash<T1, T2>, QMap<T1, T2> or QPair<T1, T2> where T1 and T2 are registered meta types
• QPointer<T>, QSharedPointer<T>, QWeakPointer<T>, where T is a class that derives from QObject
• Enumerations registered with Q_ENUM or Q_FLAG
• Classes that have a Q_GADGET macro

See also qRegisterMetaType().

Q_DECLARE_OPAQUE_POINTER(PointerType)

This macro enables pointers to forward-declared types (PointerType) to be registered with QMetaType using either Q_DECLARE_METATYPE() or qRegisterMetaType().

This function was introduced in Qt 5.0.

See also Q_DECLARE_METATYPE() and qRegisterMetaType().

Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE(Container)

This macro makes the container Container known to QMetaType as a sequential container. This makes it possible to put an instance of Container<T> into a QVariant, if T itself is known to QMetaType.

Note that all of the Qt sequential containers already have built-in support, and it is not necessary to use this macro with them. The std::vector and std::list containers also have built-in support.

This example shows a typical use of Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE():

 #include <deque>

 Q_DECLARE_SEQUENTIAL_CONTAINER_METATYPE(std::deque)

 void someFunc()
 {
     std::deque<QFile*> container;
     QVariant var = QVariant::fromValue(container);
     // ...
 }

Q_DECLARE_SMART_POINTER_METATYPE(SmartPointer)

This macro makes the smart pointer SmartPointer known to QMetaType as a smart pointer. This makes it possible to put an instance of SmartPointer<T> into a QVariant, if T is a type which inherits QObject.

Note that the QWeakPointer, QSharedPointer and QPointer already have built-in support, and it is not necessary to use this macro with them.

This example shows a typical use of Q_DECLARE_SMART_POINTER_METATYPE():

 #include <memory>

 Q_DECLARE_SMART_POINTER_METATYPE(std::shared_ptr)

 void someFunc()
 {
     auto smart_ptr = std::make_shared<QFile>();
     QVariant var = QVariant::fromValue(smart_ptr);
     // ...
     if (var.canConvert<QObject*>()) {
         QObject *sp = var.value<QObject*>();
         qDebug() << sp->metaObject()->className(); // Prints 'QFile'.
     }
 }