QML: default binding for derived property - qt

Question

I want to reimplement the following QML component (MyComp.qml) in C++:
QtObject {
property bool a: true
property bool b: false
// default: binding to a && b
property bool derived: a && b
}
Which looks like this:
class MyComp : public QObject
{
Q_OBJECT
public:
explicit MyComp(QObject *parent = nullptr);
Q_PROPERTY(bool a MEMBER a_ NOTIFY aChanged)
Q_PROPERTY(bool b MEMBER b_ NOTIFY bChanged)
Q_PROPERTY(bool derived MEMBER derived_ NOTIFY derivedChanged)
signals:
void aChanged();
void bChanged();
void derivedChanged();
private:
bool a_ {true};
bool b_ {false};
// I want to set up a default binding derived = a && b that is
// overridable, just like in the QML component; probably in
// the impl. of the default ctor
bool derived_ {true};
}
This works, except for what's described in the comment for derived.
I.e. in QML, the following instantiations should all work:
MyComp {
id: default_behavior
// should use default a && b binding for derived
}
MyComp {
id: fixed_value
derived: false
}
MyComp {
id: bind_to_other
derived: a && b && some.other.value
}

Answer 1

As I don't have any Qt environment to my avail, this will be just a rough sketch of an idea, to solve the problem.
Don't use MEMBER-Properties, but those with SETTER and GETTER
Write a slot:
void onAorBChanged() { setDerived(getA() && getB()); }
Connect that slot to the signals aChanged() and bChanged() in the Constructor.
connect(this, &MyComp::aChanged, this, &MyComp::onAorBChanged);
connect(this, &MyComp::bChanged, this, &MyComp::onAorBChanged);
Disconnect that slot from both (all) signals, once the SETTER of derived is called.
disconnect(this, 0, this, &MyComp::onAorBChanged)
Of course you need to have two setters for derived - the one you call in 2. is not supposed to disconnect the slot. Only the one you use for the Q_PROPERTY
Probably, the more fail-safe solution is, to do it in QML.
So you define your CppMyComp in C++, then you create a QML Component MyComp
// MyComp.qml
CppMyComp {
derived: a && b
}
which you then use in later:
MyComp {
id: defaul_behaviour
// derived is bound, but the binding can be easily overridden.
}
MyComp {
id: fixed_value
derived: false
}
MyComp {
id: bind_to_other
derived: a && b || someOther
}