QMetaType Class Reference
[QtCore module]

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.

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>();

See also qRegisterMetaType().