No matching function for QObject::connect

Question

I'm writing a program that send an UDP frame every 10 mS. Here's how my program is supposed to work :

I've got a client class :

//Constructor
clientSupervision::clientSupervision()
{
}

void clientSupervision::sendDataUDP(){
    //Create a frame and send it
...
}

void clientSupervision::sendDataUDPTimer(int timer){
    QTimer *tempsEnvoieTrameSupervision = new QTimer();//Create a timer
    tempsEnvoieTrameSupervision->setInterval(timer);//Set the interval

    //Mise en place des connections
    QObject::connect (tempsEnvoieTrameSupervision,SIGNAL (timeout()),this, SLOT (envoiTrameSupervision())); //Connect the timer to the function
    tempsEnvoieTrameSupervision->start();// Start the timer
}

//Call sendDataUDP
void clientSupervision::envoiTrameSupervision(){
    std::cout << "Envoi de la trame de supervision";
    sendDataUDP();
}

My header file of clienSupervision.h :

#ifndef CLIENTSUPERVISION_H
#define CLIENTSUPERVISION_H
#include <winsock2.h> // pour les fonctions socket
#include <cstdio> // Pour les Sprintf
#include "StructureSupervision.h"
#include "utilitaireudp.h"
#include <QTimer>
#include <QObject>
#include <iostream>
class clientSupervision
{
    Q_OBJECT
public:
    clientSupervision();
    void sendDataUDP();
    void sendDataUDPTimer(int timer);

public slots:
    void envoiTrameSupervision();
};

#endif // CLIENTSUPERVISION_H

Then I use this in my main :

int main(int argc, char *argv[])
{
    clientSupervision c;
    c.sendDataUDPTimer(10);
    QCoreApplication a(argc, argv);

    return a.exec();
}

I've got the error :

no matching function for call to 'QObject::connect(QTimer*&, const char*, clientSupervision* const, const char*)

I don't understand why the connect function can't find a matching function.

What should I change?

Answer 1

There can be several reasons for the issue in general:

• You do not inherit QObject.

• You do not have the Q_OBJECT macro in your class.

• You do not define the method as slot in your header file where the class is declared.

Your issue is the first which can be seen here:

class clientSupervision

You should change your code to:

class clientSupervision : public QObject
//                      ^^^^^^^^^^^^^^^^

Of course, the constructor implementation and signature would need to change, too, as follows:

explicit clientSupervision(QObject *parent = Q_NULL_PTR) : QObject(parent) { ... }

In addition, you seem to leak your QTimer instance as it does not get the parent as a parameter to the constructor.

Furthermore, the QObject:: scope is needless in your code as your class ought to inherit QObject directly or indirectly either way.

Even more, I would highly encourage you to utilize the new signal-slot syntax.

Answer 2

Another possible cause of this error is trying to connect to a slot which is overloaded. For example, this well cause the same error

QObject::connect(this,
                 &MazeWidget::MyUpdate,
                 this, 
                 &QWidget::update,
                 Qt::QueuedConnection);

But not if you explicitely cast:

QObject::connect(this,
                 &MazeWidget::MyUpdate,
                 this,
                 static_cast<void (QWidget::*)()>(&QWidget::update),
                  Qt::QueuedConnection);

Answer 3

Here's another one that snuck up on me: The class of the slot object had been forward declared in the header, but not defined in the implementation by including its header.