What is Q_PROPERTY's equivalent to QML default properties?

[Asperamanca]

I have found the "default" attribute for properties defined in QML.

I would like to make a property of a C++ Q_GADGET a default property, but I can't seem to find the right keyword in Q_PROPERTY.

[kkoehne]

@Asperamanca

It's

Q_CLASSINFO("DefaultProperty", "propertyname")

See e.g. https://doc.qt.io/qt-6.2/qtqml-referenceexamples-default-example.html

[Asperamanca]

@kkoehne Wow, that's really hidden in an obscure place in the docs...
Thanks a lot!

[kkoehne]

@Asperamanca To be fair, it's also explained in

https://doc.qt.io/qt-6/qtqml-cppintegration-definetypes.html

Where did you look for this information? Just so that we can add some cross-references ...

[Asperamanca]

@kkoehne
I looked in the explanation of the Qt Property system, and searched the whole page for "default". I also looked for links to further information especially around the section describing Q_PROPERTY

[Asperamanca]

@Asperamanca But I see, this default property is a QML-specific thing, so it doesn't belong into the general explanation of the Qt Property System. Maybe a link to the QML-specific parts would help.

[kkoehne]

@Asperamanca Created https://codereview.qt-project.org/c/qt/qtbase/+/526494 for now. let's see.

[Asperamanca]

@kkoehne However, does this even work for properties of value types?

I have something like

namespace MyModule
{

class IntWrapper
{
    Q_GADGET
    QML_VALUE_TYPE(myModule_IntWrapper)
    Q_PROPERTY(int intValue READ getIntValue WRITE setIntValue)
    Q_CLASSINFO("DefaultProperty", "intValue")
public:
  int m_Value = {};
  int getValue() const;
  void setValue(const int& arg);
};
} // namespace MyModule

And in QML:

import MyModule;

ComboBox
{
  readonly property myModule_IntWrapper testInt: highlightedIndex
}

I would expect testInt to be initialized with the highlightedIndex, but instead I get
Unable to assign int to MyModule::IntWrapper.

I can do this:

ComboBox
{
  property myModule_IntWrapper testInt: highlightedIndex
  onHighlightedIndexChanged: testInt.intValue  = highlightedIndex
}

...but that's not very elegant.

[Asperamanca]

@Asperamanca
So I solved this using an undocumented feature:

namespace MyModule
{

class IntWrapper
{
    Q_GADGET
    QML_VALUE_TYPE(myModule_IntWrapper)
    Q_PROPERTY(int intValue READ getIntValue WRITE setIntValue)
    QML_STRUCTURED_VALUE
public:
  Q_INVOKABLE IntWrapper(const int& value);
  int m_Value = {};
  int getValue() const;
  void setValue(const int& arg);
};
} // namespace MyModule

EDIT: This is going to be documented in Qt 6.7, see https://bugreports.qt.io/browse/QTBUG-113490?jql=text ~ "QML_STRUCTURED_VALUE"