Can't conect two frames with custom signals

Question

I'm trying to connect two frames with a custom signal but I'm not really getting it.

This code is just an example of what im trying to do in my program, my objective is to transfer data between frames.

Files:

(sender)

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H

    #include <QMainWindow>

    namespace Ui {
    class MainWindow;
    }

    class MainWindow : public QMainWindow
    {
        Q_OBJECT
    signals:
        void send();

    public:
        explicit MainWindow(QWidget *parent = nullptr);
        ~MainWindow();

    private:
        Ui::MainWindow *ui;
    private slots:
        void on_pushButton_clicked();
    };

    #endif // MAINWINDOW_H

On "mainwindow.cpp" I've got the void on_pushButton_clicked() that emits the signal and shows the new frame:

private slot void:

        void MainWindow::on_pushButton_clicked()
    {
        emit send();
        Dialog sw;
        sw.setModal(true);
        sw.exec();
    }

(receiver):

        #ifndef DIALOG_H
    #define DIALOG_H

    #include <QDialog>
    #include <QDebug>

    namespace Ui {
    class Dialog;
    }

    class Dialog : public QDialog
    {
        Q_OBJECT

    public:
        explicit Dialog(QWidget *parent = nullptr);
        ~Dialog();

    private slots:
        void receive();

    private:
        Ui::Dialog *ui;
        int a;
    };

    #endif // DIALOG_H

and the .cpp:

        #include "dialog.h"
    #include "ui_dialog.h"
    #include "mainwindow.h"


    Dialog::Dialog(QWidget *parent) :
        QDialog(parent),
        ui(new Ui::Dialog)
    {
        ui->setupUi(this);
        a=0;
        MainWindow w;
        connect(&w, SIGNAL(send()), this, SLOT(receive()));
        qDebug() << a;
    }


    Dialog::~Dialog()
    {
        delete ui;
    }

    void Dialog::receive(){
        qDebug() << "ola";
        a++;
    }

Conclusion: So basicly the function Dialog doesn't print the qDebug(), and 'a' is still 0, so I conclude that the connection isn't set/executed.

Thanks all, Best regards, Dylan Lopes.

edit: Wrote a conclusion on the end of the post.

Sorry, forgot to mencion it, i edited a conclusion on the end of the post. — Dylan Lopes
'On "mainwindow.cpp" i've got the void on_pushButton_clicked() that emits the signal and shows the new frame,' — Dylan Lopes
Your object "MainWindow w" was created in the constructor of the Dialog so when it go out of the scope of the view it will be deleted, so try to move it to private section of the class and I think all'll be ok — Alexander Chernin

G.M. G.M. · Accepted Answer · 2018-11-26T13:01:21

Consider the code in your Dialog constructor...

MainWindow w;
connect(&w, SIGNAL(send()), this, SLOT(receive()));

This creates a locally scoped MainWindow on the stack and connects its send() signal to the Dialog's receive() slot. But the MainWindow -- and, hence, the connection -- will be destroyed as soon as the Dialog constructor has completed.

In addition, looking at MainWindow::on_pushButton_clicked...

void MainWindow::on_pushButton_clicked()
{
    emit send();
    Dialog sw;
    sw.setModal(true);
    sw.exec();
}

You emit the send() signal before constructing the Dialog.

I don't really know enough about what you're trying to achieve to provide a definitive answer, but in the interests of getting some kind of signal/slot interaction you might want to do the following: change the Dialog constructor to...

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
    , ui(new Ui::Dialog)
    {
        ui->setupUi(this);
        a=0;
        qDebug() << a;
    }

And change MainWindow::on_pushButton_clicked to...

void MainWindow::on_pushButton_clicked()
{
    Dialog sw;
    connect(this, &MainWindow::send, &sw, &Dialog::receive);
    emit send();
    sw.setModal(true);
    sw.exec();
}

That should at least result in Dialog::receive being invoked and you can work from there.

Can't conect two frames with custom signals

2 Answers