Qt. Dialog and main windows location

[born in USSR]

I'm new in Qt. I create main and dialog windows in Qt4 Designer. I want
to place dialog window in the middle of main window, and place main
window in the center of screen. Is it possible?
require 'Qt4'
require 'mainwindow.rb'
require 'logindialog.rb'

class MW < Qt::MainWindow
slots 'setfocus()'
def initialize parent=nil
super
@ui = Ui_MW.new
@ui.setupUi self
...
end
end

class LoginDL < Qt:ialog
slots 'enter_click()'
def initialize parent=nil
super
@ui = Ui_LoginDL.new
@ui.setupUi self
...
end
...
end

if $0 == __FILE__
app = Qt::Application.new(ARGV)
$mainw = MW.new
$mainw.show

loginw=LoginDL.new
loginw.show
app.exec
end