I get a run time exception

Then run through debugger to see where and why exactly it is crashing...
I guess it is crashing inside ScanFinished() slot.

Connecting a signal to another signal is perfectly valid.

now if I uncomment
connect(&bt_device, &Device::scanFinished, this, &BT_DEVICE::ScanFinished);
I get a run time exception

The inferior stopped because it received a signal from the operating system.

in devise.h

signals:
    void scanFinished();

in my h

public slots:
    void ScanFinished();

seems OK.

So this is wrong

Device::Device()
{
    connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::scanFinished);
}

I connect a signal to a signal.

those Q_PROPERTY - do I need it?

If you don't use them then you don't need them.

class Device: public QObject
{
    Q_OBJECT
    Q_PROPERTY(QVariant devicesList READ getDevices NOTIFY devicesUpdated)
    Q_PROPERTY(QVariant servicesList READ getServices NOTIFY servicesUpdated)
    Q_PROPERTY(QVariant characteristicList READ getCharacteristics NOTIFY characteristicsUpdated)
    Q_PROPERTY(QString update READ getUpdate WRITE setUpdate NOTIFY updateChanged)
    Q_PROPERTY(bool useRandomAddress READ isRandomAddress WRITE setRandomAddress NOTIFY randomAddressChanged)
    Q_PROPERTY(bool state READ state NOTIFY stateChanged)
    Q_PROPERTY(bool controllerError READ hasControllerError)

//and so on

those Q_PROPERTY - do I need it? it's a QML related stuff.
I took the class from a QML project.

On Linux the same 'vtable' problem

Exact same code?
Do a full rebuild.

On Windows I get a different error - error: multiple definition of `Device::scanFinished()' in moc_device.cpp.

Just realised: this is in Device class.
Please show Device code (header and cpp).
You probably defined scanFinished() in cpp file, you should not do that for signals!

@jenya7 said in Qt Bluetooth - no devices discovered.:

I wonder how can it be - moc file is a generated file after build

It is probably a remnant from previos version when scanFinished was declared as signal.
Do a complete rebuild: delete build folder, run qmake and build.

Deleted all from the build folder. Clean. Rebuild. I see new files created. The same problem.
Renamed scanFinished for scanCompleted. Still the same problem.

I wonder how can it be - moc file is a generated file after build

It is probably a remnant from previos version when scanFinished was declared as signal.
Do a complete rebuild: delete build folder, run qmake and build.

@jenya7 said in Qt Bluetooth - no devices discovered.:

Is it OK?

Why did you comment out the connect?

for debug. I still get the error.
On Windows I get a different error - error: multiple definition of `Device::scanFinished()' in moc_device.cpp.
first defined here device.cpp.
I wonder how can it be - moc file is a generated file after build.

Is it OK?

Why did you comment out the connect?

class BT_DEVICE : public QObject
{
    Q_OBJECT

    public:
    BT_DEVICE(QObject *parent = nullptr);
    void BLE_Scan();

public slots:
    void ScanFinished();
};

cpp

BT_DEVICE::BT_DEVICE(QObject *parent) : QObject(parent)
{
    //for debug
    //connect(&bt_device, &Device::scanFinished, this, &BT_DEVICE::ScanFinished);
}

void BT_DEVICE::BLE_Scan()
{
   scanning = 1;

   bt_device.startDeviceDiscovery();
}

void BT_DEVICE::ScanFinished()
{
    QVariant devices = bt_device.getDevices();
}

Is it OK?

while (scanning) ;

Not related to your current problem: this is a no go in an event driven application! You are blocking the event loop! This means that no signals will be handled. Redesign this part of your app.

Back to your current problem: I did a mistake above. BT_DEVICE::ScanFinished should be a slot of course.

class BT_DEVICE : public QObject
{
    Q_OBJECT

    public:
    BT_DEVICE(QObject *parent = nullptr);
    void BLE_Scan();

signals:
    void ScanFinished();
};

and cpp file

BT_DEVICE::BT_DEVICE(QObject *parent) : QObject(parent)
{
    connect(&bt_device, &Device::scanFinished, this, &BT_DEVICE::ScanFinished);
}

void BT_DEVICE::BLE_Scan()
{
   scanning = 1;

   bt_device.startDeviceDiscovery();

   while (scanning) ;

   QVariant devices = bt_device.getDevices();
}

void BT_DEVICE::ScanFinished()
{
    scanning = 0;
}

#include <QObject>

class BT_DEVICE : public QObject
{
Q_OBJECT

signals:
    void scanFinished();
...

Do a complete rebuild of your project after these changes.

BT_DEVICE::BT_DEVICE(QObject *parent) : QObject(parent)
{
    connect(&bt_device, &Device::scanFinished, this, &BT_DEVICE::ScanFinished);
}

I get - "undefined reference to `vtable for BT_DEVICE' ". And I don't have any virtual function.

What if I do deviceScanFinished public?

To achieve what exactly? deviceScanFinished is a slot not signal in Device - how will making it public help other classes?

"The signal already set" - the signal from private member is "set", this does not help other classes as you already discovered.

Why don't you try my approach? Signals/slots are one of the most important features in Qt and it is really not much work:

class Device...
{
signals:
    void scanFinished();
}

// In Device constructor:
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::deviceScanFinished);
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::scanFinished);

// Now in some other class you simply do:
connect(&bt_device, &Device::scanFinished, this, &SomeOtherClass::someSlot);

Isn't this easy and clear? And you do not expose any private members from Device to outer world (which would be bad design).

OK. I see what's going on. Just a strong encapsulation.

The signal already set

connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::deviceScanFinished);

and it sets m_deviceScanState

void Device::deviceScanFinished()
{
    emit devicesUpdated();
    m_deviceScanState = false;
    emit stateChanged();
    if (devices.isEmpty())
        setUpdate("No Low Energy devices found...");
    else
        setUpdate("Done! Scan Again!");
}

so my

 while (bt_device.state()) ;

actually tests the memeber

bool Device::state()
{
    return m_deviceScanState;
}

so deviceScanFinished never occurs.

@jenya7 And why is deviceScanFinished private?
It is actually apparent that it should be public...

it's an official example in the examples folder (C:\Qt\Examples\Qt-5.12.0\bluetooth\lowenergyscanner).
I wouldn't dare to change it, only if you say so.

connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Device::scanFinished); // scanFinished is the new signal