Comparing QString causes the program to crash .

[ladiesfinger]

Hi all ,
i designed a form using Qt designer and compiled it with uic to produce
ui_mainwindow.h
The other files are :
1. main.cpp
2. mainwindow.cpp
3. mainwindow.ui
4. mainwindow.h ( default files )

i've included ui_mainwindow file in header file.
In my form , among many other fields , i want to get username and password from QLineEdit .
When the user press login pushbutton , it should check the name and password manually entered in the corresponding slot invoked .
here's the slot's func:

void MainWindow::on_login_clicked()
{
user=usernameip->displayText();        // usernameip is lineedit's name 
pass=passwordip->displayText();       //         ''                ''
    if(user.compare("user")&&pass.compare("pass"))
    {
        //i wanted to invoke another dialog from here but i cant put even a msgbox
        //'coz the program crashes bfore entering this
    }

void MainWindow::on_login_clicked()
{
user=usernameip->displayText();        // usernameip is lineedit's name 
pass=passwordip->displayText();       //         ''                ''
    if(user.compare("user")&&pass.compare("pass"))
    {
        //i wanted to invoke another dialog from here but i cant put even a msgbox
        //'coz the program crashes bfore entering this
    }

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Both user and pass are declared as QString in header file.
If u need the code for other files , ill post it immediately .
i spent about some 3 hours without success , this is my first prog ...