Add virtual QObjectPrivate::writeToDebugStream

QObject by default writes class- and objectName to the debug stream,
but QObject subclasses might want to write different, or additional
information. Providing class-specific debug stream operators is not a
good solution for this, as it doesn't respect runtime polymorphism.

With a virtual helper in QObjectPrivate, class authors can add an
override.

Change-Id: I382126ae5a56391d0c7e5ccb58fbbafa6aab5c77
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/kernel/qobject.cpp

@@ -4472,15 +4472,23 @@ void QObject::dumpObjectInfo() const
 
 
 #ifndef QT_NO_DEBUG_STREAM
+void QObjectPrivate::writeToDebugStream(QDebug &dbg) const
+{
+    Q_Q(const QObject);
+    dbg.nospace() << q->metaObject()->className() << '(' << (const void *)q;
+    if (!q->objectName().isEmpty())
+        dbg << ", name = " << q->objectName();
+    dbg << ')';
+}
+
 QDebug operator<<(QDebug dbg, const QObject *o)
 {
     QDebugStateSaver saver(dbg);
     if (!o)
         return dbg << "QObject(0x0)";
-    dbg.nospace() << o->metaObject()->className() << '(' << (const void *)o;
-    if (!o->objectName().isEmpty())
-        dbg << ", name = " << o->objectName();
-    dbg << ')';
+
+    const QObjectPrivate *d = QObjectPrivate::get(o);
+    d->writeToDebugStream(dbg);
     return dbg;
 }
 #endif

src/corelib/kernel/qobject_p.h

@@ -188,6 +188,10 @@ public:
 
     virtual std::string flagsForDumping() const;
 
+#ifndef QT_NO_DEBUG_STREAM
+    virtual void writeToDebugStream(QDebug &) const;
+#endif
+
     QtPrivate::QPropertyAdaptorSlotObject *
     getPropertyAdaptorSlotObject(const QMetaProperty &property);