Private QObject API

There are certain QObject macros and conventions that are for use in private Qt APIs only. This page describes them.

Q_PRIVATE_PROPERTY

Introduced here.

This macro is used when you have a C++ class that is public that exposes a property whose type is not public. For example, QQuickItem is a public C++ class that exposes an anchors property, whose type is the private QQuickAnchors class:

   Q_PRIVATE_PROPERTY(QQuickItem::d_func(), QQuickAnchors * anchors READ anchors DESIGNABLE false CONSTANT FINAL)

In this case, the macro expects there to be a getter named anchors() in the type returned by QQuickItem::d_func(), which there is:

   QQuickAnchors *anchors() const;

When using this macro, it's necessary to include the generated moc file at the end of the corresponding .cpp file:

   QT_END_NAMESPACE
   
   // ...
   
   #include "moc_qquickitem_p.cpp"

If you don't do this, you may see an error like the following (example taken from another class -- replace popup with item and it's the same error):

   [...] moc_qquickpopup_p.cpp
   .moc\debug\moc_qquickpopup_p.cpp(1069): error C2027: use of undefined type 'QQuickPopupPrivate'
   c:\dev\qt5-dev-d\qtquickcontrols2\src\quicktemplates2\.moc\debug\../../../../../../qt5-dev/qtquickcontrols2/src/quicktemplates2/qquickpopup_p.h(71): note: see declaration of 'QQuickPopupPrivate'
   .moc\debug\moc_qquickpopup_p.cpp(1069): error C2227: left of '->anchors' must point to class/struct/union/generic type

Q_PRIVATE_SLOT

Similar to Q_PRIVATE_PROPERTY, Q_PRIVATE_SLOT is used when you have a slot in a private class that you want to be considered part of the API of the corresponding public class. Using QQuickItem as an example again, Q_PRIVATE_SLOT is used to declare this private slot in qquickitem.h:

   private:
       Q_PRIVATE_SLOT(d_func(), void _q_resourceObjectDeleted(QObject *))

The slot itself is declared in qquickitem_p.h:

   void _q_resourceObjectDeleted(QObject *);