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How to solve Object::connect: No such signal

  • I'm trying to display a sequence of images coming at 30 image per second in qt label but I'm getting that error of signal. This is my code:

    class MainWindow : public QMainWindow {
      MainWindow(int argc, char** argv, QWidget *parent = {});
      ~MainWindow() override;
      void callBackColor(const sensor_msgs::ImageConstPtr& msg);
      void setImage(const QImage &);
      Q_SIGNAL void newImage(const QImage &);
      Ui::MainWindowDesign ui;
      ros::Subscriber sub;
    MainWindow::MainWindow(int argc, char** argv, QWidget *parent)
        : QMainWindow(parent)
      QObject::connect(this, SIGNAL(&MainWindow::newImage(const QImage &)), this, SLOT(&MainWindow::setImage(const QImage &)));
      ros::NodeHandle n;
      sub = n.subscribe("/usb_cam/image_raw", 1, &MainWindow::callBackColor, this);
    void MainWindow::setImage(const QImage &image) {
      atui pix = QPixmap::fromImage(image);
    void MainWindow::callBackColor(const sensor_msgs::ImageConstPtr& msg)
      try {
        auto cv_ptr = cv_bridge::toCvCopy(msg, sensor_msgs::image_encodings::BGR8);
        QImage temp(&(msg->data[0]), msg->width, msg->height, 
        Q_EMIT newImage(temp);
      catch (cv_bridge::Exception& e) {
        ROS_ERROR("cv_bridge exception: %s", e.what());

    I get that error Object::connect: No such signal qdude::MainWindow::&MainWindow::newImage(const QImage&*)
    and I tried to save QImage temp in a file and I get the frames I want. So the issue is as the error is saying, there's no signal!
    Do you have any idea where could be the issue?

  • Moderators

    You are mixing old and new connect syntax.

    Either use this:

    // Old syntax
    connect(this, SIGNAL(MainWindow::newImage(const QImage)), this, SLOT(MainWindow::setImage(const QImage)));

    Or this:

    // New syntax:
    connect(this, &MainWindow::newImage, this, &MainWindow::setImage);

    I recommend using new syntax whenever possible.

  • This post is deleted!

  • @sierdzio I was using the new version but I get the error of matching error: no matching function for call to ‘qdude::MainWindow::connect(qdude::MainWindow*, void (qdude::MainWindow::*)(const QImage&), qdude::MainWindow*, void (qdude::MainWindow::*)(const QImage&))’

  • Moderators

    @jaouad100 said in How to solve Object::connect: No such signal:

    void setImage(const QImage &);

    Make this method as slot:

    protected slots:
      void setImage(const QImage &);

  • @sierdzio The same issue :/

  • @jaouad100 said in How to solve Object::connect: No such signal:

    @sierdzio I was using the new version

    If you say you are using the "new version" syntax, we would not expect to see SIGNAL() or SLOT().

    Would you care to either confirm your code is still exactly as shown in your first post, or update it correctly?

  • @JonB I confirm that the code posted is the version I'm working on at the moment

  • @jaouad100
    Then I think (I am not a C++-er) you'll find that as @sierdzio wrote for the old syntax you need to remove the two & characters prior to MainWindow:: in both SIGNAL() & SLOT() (because they effectively put their own & in for you) ....

  • @JonB Yes I noticed that but it is not the issue

  • @jaouad100
    If it is not the issue, then why does your error message read:
    qdude::MainWindow::&MainWindow::newImage(const QImage&*)
    with that middling & in it? I believe it is....

    If I wanted to use the old syntax, I would be writing:

    QObject::connect(this, SIGNAL(newImage(const QImage &)), this, SLOT(setImage(const QImage &)));

    exactly as per e.g.

  • @JonB Yes you're right. It was not only the reference because I tried without the reference but it didn't work but also the qualification on the names MainWindow:: I took it off and now it works, thanks!

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