How to make a dialog window always on the front at my application level

If you can't send parent to your dialog_window (based on code conditional) Like me, use flags.

Dialog_dlg::Dialog_dlg(QWidget *parent)
    : QDialog(parent), ui(new Ui::Dialog_dlg)
{
    ui->setupUi(this);
    setFixedSize(width(), height()); //for no maximaize
    setWindowFlag(Qt::WindowStaysOnTopHint);
}

I used this and good to me. See other flags, test them, and choose.

You can achieve this by giving the dialogs a parent. A child dialog always stays on top of its parent window.