class MyLineEdit: public QLineEdit
{
Q_OBJECT
public:
    MyLineEdit(const QString & title, QWidget * parent = 0);

public slots:
    void onTextChanged()
    {
        QToolTip::showText(QPoint(100,100), "test tooltip...., this is the example of tooltip dismissed immediately");
    }
};

MyLineEdit::MyLineEdit(const QString & title, QWidget * parent): QLineEdit(title, parent)
{
    QObject::connect(this, SIGNAL(textChanged(const QString&)), this, SLOT(onTextChanged()));
}

int main (int argc, char * argv[]) {
    QApplication app(argc, argv);
    MyLineEdit* le = new MyLineEdit("test LineEdit");
    le->show();
    return app.exec();
}