How To Connect Emitter From MainWindow To AnotherWindow

Eshya

I have a main program in MainWindow whose computational results I emit to SecondWindow

mainwindow.h

private:
   secondwindow *_secondwindow;
signals:
    void sendLabel(QString stringDisplay);

secondwindow.h

private:
   MainWindow *_mainW;
private slots:
    void changeDisplay(QString display);

mainwindow.cpp

void MainWindow::secondwindow(){

   _secondwindow = new secondwindow(this)
    _secondwindow->showMaximized();

    emit sendLabel("400");


}

secondwindow.cpp

secondwindow::secondwindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::secondwindow)
{
    ui->setupUi(this);
    connect(mainW, SIGNAL(sendLabel()),this,SLOT(changeDisplay()));

}

void secondwindow::changeDisplay(QString display){
     ui->lcdNumber->display(display);
}

But I get an error because the main window is not recognized

error: ‘MainWindow’ does not name a type; did you mean ‘QMainWindow’?
   27 |     MainWindow *_mainW;
      |     ^~~~~~~~~~
      |     QMainWindow

What is the correct way to emit variable from main window to another window ?

jsulm

@Eshya said in How To Connect Emitter From MainWindow To AnotherWindow:

private:
MainWindow *_mainW;

You do not need a pointer to MainWindow in second window! A child should usually not know anything about its parent.
The connection should be done in MainWindow after the creation of the second window instance.

Eshya

@jsulm Ok, but when i try to do this I got error force closed when show second window

    _secondwindow->showMaximized();
    connect(mainW, SIGNAL(sendLabel()),_secondwindow,SLOT(changeDisplay()));
    emit sendLabel("400");

or

QObject::connect: No such signal MainWindow::sendLabel() in

jsulm

@Eshya said in How To Connect Emitter From MainWindow To AnotherWindow:

I got error

Of course you get an error: please check the signature of your signal and what you have in your connect - they do not match!
And you also should use the new Qt5 connect syntax.

Eshya

@jsulm said in How To Connect Emitter From MainWindow To AnotherWindow:

Qt5 connect syntax.

Okay, thank you. Silly me because not yet updated

connect(this,&MainWindow::sendLabel,_secondwindow,&SecondWindow::changeDisplay);

I just need to change thic section and done. Thank you