Unsolved Implementation Singleton problem

[Creatorczyk]

Hi,
I wrote class SMessenger which is a singleton but during compilation i get error:
undefined reference to 'SMessenger::instance_'

This is my code

class SMessenger : public QMqttClient
{
    Q_OBJECT
public:
    static SMessenger* instance(){
        if(!instance_) instance_ = new SMessenger();
        assert(instance_ != NULL);
        return instance_;
    }
public slots:
    void sendMsgToDevice(QString topic, bool state);

protected:
    SMessenger(QObject *parent = nullptr);
private:
    SMessenger(SMessenger const&);
    SMessenger& operator=(SMessenger const&);

    static SMessenger* instance_;
    QMqttClient *m_mqttClient;

private slots:
    void slotErrorChanged(const QMqttClient::ClientError e);
    void slotStateChanged();
    void slotDisconnected();
    void slotConnected();

signals:
    void signalNewParamsFromGreenhouse(Current_parameters parameters);
};

The problem is in the method instance()

[Chris Kawa]

You're missing a definition of your static variable. Put that in your .cpp file:

SMessenger* SMessenger::instance_ = nullptr;

Btw. don't forget to delete your singleton before your application object is destroyed. No QObject should live longer than that. One way to do it is to connect QApplication::aboutToQuit signal to your singleton's deleteLater slot when you're creating it.

Btw.2 Don't use NULL. It's an old C style macro. If you need to be that explicit use nullptr or you could just assert(instance_).