How to emit a Qt signal from inside a function pointer? - c++

Question

While implementing the interface to a device there is code like the following in MyClass constructor
pDeviceInstance = new Device()
pDeviceInstance->loadLibrary();
pDeviceInstance->Create(functionpointer)
There are asyncronous events comming out and calling functionpointer (which I would like to deal with).
Both functionpointer and pDeviceInstance are static.
I would like to emit a Qt signal from inside functionpointer but because it is static its not possible to emit the signal.
I tried:
To make functionpointer a lambda in the hope to emit signals from inside the lambda.
But for that to happen lambda needs to capture this,
and in order for the lambda to decay to a function pointer it cannot capture anything.
To make functionpointer non static but in that case it seems to be a function pointer of MyClass
which is different type.
So, I'm running out of options in my plan of emit a Qt signal from inside that functionpointer.
Is there anything I'm missing here?

Answer 1

Not a problem at all. Hook up a QObject with the signals you desire as a child of qApp (QApplication::instance), and then use it via the global qApp pointer:
class MySignaler : public QObject {
Q_OBJECT
public:
MySignaler(QObject *parent = nullptr) : QObject(parent) {
setParent(qApp);
}
Q_SIGNAL void deviceActivity();
static bool emitDeviceActivity() {
if (!qApp) return false;
auto *self = qApp->findChild<MySignaler*>();
if (!self) return false;
emit self->deviceActivity();
return true;
};
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
MySignaler signaler;
Device device;
device.loadLibrary();
device.create(&MySignaler::emitDeviceActivity);
// ...
}