Proper way to open a QDialog

[casperbear]

Hello! I'm trying to figure out a way to properly open a QDialog immediately after QMainWindow is shown. In examples below

class WMain : public QMainWindow
...
class WNew : public QDialog

So far I've tried three approaches:
1)

void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    WNew window(this);
    window.exec();
}

QMainWindow is not shown before QDialog is shown. Internet says it's because QDialog::exec is a synchronous call. Changing showEvent to the first paintEvent doesn't solve the problem (QMainWindow is shown before QDialog is shown, but some QMainWindow controls may be black). Using timer-based solutions doesn't look right.
2)

void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    WNew window(this);
    window.open();
}

QDialog is not shown at all. I assume it's because QDialog::open is an asynchronous call and WNew destructor is called when code exits showEvent function.
3)

void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    WNew *window = new WNew(this);
    window->open();
}

This seems to work, but there is a memory leak. WNew destructor is not called when the dialog is closed. Can you suggest please where I should call the destructor? Or some different approach?

[SGaist]

Hi,

Can you explain your use case ?

As for the memory leak, you can set the ”WA_DeleteOnClose” widget attribute.

[mpergand]

You can use show() :

void show()
        {
        QMainWindow::show();
        WNew dlg(this);
        dlg.exec();
        }

[casperbear]

My use case is that I have a dialog for creating a new document. This dialog should be initiated by a user click and also opened on application start.
WA_DeleteOnClose worked for me. Thank you. I would have never guessed this attribute.

It's particularly convenient that I can programmatically control when to use it. (Use it when dialog object is created in heap and don't use it when dialog object is created in stack)

// on application start
void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    WNew *window = new WNew(this);
    window->setAttribute(Qt::WA_DeleteOnClose);
    window->open();
}

// on user click
void WMain::on_actionNew_triggered()
{
    WNew window(this);
    window.exec();
}

[SGaist]

The you should rather use a single shot QTimer with 0 as timeout value at the end of your constructor to call a slot that will show your dialog. This way the event loop will have started properly and your WMain object should be visible.

[casperbear]

SGaist, yes this approach is even better. It solved problem with dialog centering (alignment relative to parent) and initial dialog 'active' status. I wouldn't come up with '0' argument in a lifetime. I thought about very small numbers but not '0'. :) I guess '0' works because (if I recall) timer events are processed in event loop only when there are no other events (in win32 at least).

void WMain::showEvent(QShowEvent *event)
{
    QMainWindow::showEvent(event);

    QTimer::singleShot(0, [this]() {
        WNew window(this);
        window.exec();
    });
}