I went throught suggested "questions" about my problem. However neither does not solve it.

I program two windows. The second window is opening from first window. I need active the both windows, however to start the first window(MainWindow) I use:

    int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.setWindowModality(Qt::NonModal);        
    return a.exec();
}

As was mentioned, the second window is started from pushButton, which is situated in first window(MainWindow) by same way.

void MainWindow::on_pushButton_2_clicked()
{    
    Graphics gr;
    gr.setWindowModality(Qt::NonModal);
    gr.exec();
}

I changed modality to NonModal,however the problem is without change. The Non-Modal mean:"The window is not modal and does not block input to other windows." <- from documentation By documentation is recommended to avoid used .exec(). The alternatives are .show() and open(), which i tried. After the modification, the second window is shut down immediately after opening. after open immediately shut down.

Do you have any idea, how to solve that?

CodePudding user response：

Graphics gr; defines a local variable, so the object is destructed as soon as it goes out of scope (at the end of your function).

In Qt, the typical approach is to work with pointers to Qt widgets (and, more generally, QObjects), but have a parent for each – the parent will clean it up.

Try this instead:

auto gr = new Graphics(this);
gr->setWindowModality(Qt::NonModal); // this is the default, no need for this call
gr->show();

This way the window will survive until your main window is destructed. In Qt there is also a mechanism for widgets/objects to self-destruct:

this->deleteLater();

So for example if you want to close and cleanup gr you can use that mechanism (you could also store gr as a member and call gr->deleteLater().