void MainWindow::on_saveButton_clicked()
{
    QString stName;
    int stNum;
    float stAve;
    stName = ui->nameEdit->text();
    stAve = ui->aveEdit->text().toFloat();
    stNum = ui->stEdit->text().toUInt();
    st.setName(stName);
    st.setAverage(stAve);
    st.setStNo(stNum);
    QFile file("student.dat");
    if(!file.open(QIODevice::Append | QIODevice::WriteOnly | QIODevice::Text)) {
        QMessageBox::information(this, "Warning", "File not open");
        return;
    }
    QDataStream ds(&file);
    ds << st;
    file.flush();
    file.close();
    clearForm();
}
