right way to open a new window

[wind]

First when the application start I display the login screen. Then after successful login i have to display the main window. What is the right way to close the login screen and display the main window?

At the moment i have something like this:

main_window *main = new main_window();			
this->close(); // close login			
main->show();	// show main

main_window *main = new main_window();			
this->close(); // close login			
main->show();	// show main

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When i call main_window *main = new main_window(this), then the application is not working right.

I have searched Qt examples & demos but all use just one window.