Login window

  • Hi, I'm trying to do a login window wich must to be closed and open the main window. When user achieves the login I do:

    @MainWindow mw;

    But the new window does not appear. What am I doing wrong or what do I have to do?.

    Thank you.

  • if you are creating window inside some function then it is wrong as the window get created and shown, immediately it will get destroyed as the "mw" scope is inside that function only.
    int main(...)
    QApplication app(...);
    Mainwindow mw;
    //wrong method
    return 0;
    // correct method
    return app.exec();
    /* here it will keep running as a thread so you will able to view the mainwindow, if return is some 0,1 etc. then it will break and you will not able to see the windows created, as it got destroyed out side the scope*/

  • There's two simple ways:

    1. Open your Login-Widget in main() before showing MainWindow (both from main())
    2. Show MainWindow and Open the Login-Widget from inside MainWindows constructor. (MainWindow itself will not be displayed until the constructor is finished)

  • Finally I did:

    @MainWindow* mw = new MainWindow;

    Creating the window at the heap works. Thank you for the answers.

  • @ninio:could you post your code here,. it might be of some help to me. Thanks.

  • Hi Ninio,
    Though it might work like that you have a potential memory leak at hand. When the new MainWindow is created it will give you the pointer to it so you can reverence it later on in your program. If the new operator is inside your logInWindow the pointer will be destroyed when the LogInWindow function is closed making the MainWindow unreachable by your code! The mw pointer should then be a public MainWindow pointer.
    When the pointer is not accessable anymore the memory can't be released until the program shutdown. When a new logIn Window is created a complete new mainwindow will appear.
    Might be better to use a parent pointer in the LogInWindow to alter the mainWindow show/hide options.

  • Actually, the window will be accessible via the QApplication object. You are right in principle, but things are not as bad as they seem :-)

  • Ah, oke, learned something new there. Nice, but still it is bad pratice to create a new object and don't keep the pointer at hand ;-)

  • Hi all,
    I tried to resolve the problem by writing a code example: (Connecting to a PostgreSQL data base)

    create a login window

    create a data base connection

    show the login window

    enter the username and the passwd (repeat until the login is accepted or the operation is canceled)

    login succeeded ==> create and show the main window

    int main(int argc, char *argv[])
    QApplication app(argc, argv);

    LoginWindow Lwindow;

    /* accepted = 1; rejected = 0; */
    int result;
    bool pass = false;
    int response;

    QSqlDatabase Dbase;
    Dbase = QSqlDatabase::addDatabase("QPSQL");

    result = Lwindow.result();
    if(result == 0)
    pass = true;
    return 0;
    QString user = Lwindow.getUser();
    QString password = Lwindow.getPassword();
    pass = Dbase.open();

    /* if(m_dbase.lastError().isValid()) */
    response = QMessageBox::critical(NULL, "Data Base Access Problem",Dbase.lastError().text(),QMessageBox::Cancel|QMessageBox::Retry);
    if(response == QMessageBox::Cancel)
    pass = true;
    return 0;

    MainWindow Mwindow(Dbase);
    return app.exec();


  • [quote author="issam" date="1348049464"]Hi all,
    I tried to resolve the problem by writing a code example like this :

    Nice. And what was the result of that attempt? Did it work? If it did: congrats. If it did not: what failed exactly? What did you expect?

  • If you ask me it should work just fine. Maybe a bit of comments to add, but looks fine to me. Did it work??

  • It works well ! ;)


  • In that case, please go and edit your first post in this topic to add a [Solved] tag to the title.

  • [quote author="issam" date="1348128467"]
    It works well ! ;)


    Thank you....

  • Dear issam,

    Where is the Lwindow definition? Do you separate into lwindow.cpp and lwindow.h?

    Thanks in advance.

  • Yes in loginwindow.h and LoginWindow.cpp :)

  • Can you reupload the source code :) ?

Log in to reply