senderSignalIndex() not accessible

Redman

class IA : public QObject {
Q_OBJECT
.
.
.
};

class A : public IA {
Q_OBJECT
.
.
.
};

class IB : public QObject {
Q_OBJECT
.
.
.
};

class B : public IB {
Q_OBJECT
.
.
.
};

B::slotForSignalFromA() {
sender()->senderSignalIndex() <-- C2248
qobject_cast<A*>(sender())->senderSignalIndex() <-- C2248
qobject_cast<IA*>(sender())->senderSignalIndex() <-- C2248
}

error: C2248: 'QObject::senderSignalIndex': cannot access protected member declared in class 'QObject'

What am I missing

Redman

@JonB Simply calling
senderSignalIndex() was sufficient to get the correct signalIndex.

SGaist

Hi,

As the error message says, you are trying to access a protected member of class outside of it.

Redman

But technically im in QObject, since B inherits from it

JonB

This post is deleted!

JonB

@Redman
But you are trying to access protected members via sender()->.... You can only access protected members directly inside the derived class (i.e. on this), not on some other object. You could do it inside classes A or IA, but not from B. Either do the work inside A/IA instead, or have them export something public which calls senderSignalIndex() for you.

Redman

@JonB I see. I updated my class to this now:

class IA : public QObject {
Q_OBJECT
.
.
.
   qint32
   signalIndex() const {
      return senderSignalIndex();
   }
};

and then I try to access it like this:

   IA* a = qobject_cast<IA*>(sender());
   qint32 signal = a->signalIndex();

Which always returns -1.

JonB

@Redman
Well, I only know about correcting you on the C++ issue you asked about. I know nothing about the method. I note the docs include:

Returns the meta-method index of the signal that called the currently executing slot, which is a member of the class returned by sender(). If called outside of a slot activated by a signal, -1 is returned.

Warning: The return value of this function is not valid when the slot is called via a Qt::DirectConnection from a thread different from this object's thread. Do not use this function in this type of scenario.

I don't know whether reading https://stackoverflow.com/questions/27476554/how-to-find-out-from-the-slot-which-signal-has-called-this-slot might be relevant to you.

Redman

@JonB Simply calling
senderSignalIndex() was sufficient to get the correct signalIndex.

GrecKo

note that sender(), senderSignalIndex() are quite expensive calls (they involves a mutex) and can most of the time be refactoring to connect to a lambda capturing the calling context.