char znak[3]={'+', '-', '*'};

connect(ui.sumButton, SIGNAL(clicked()), this, SLOT(getAndshowResult(znak[0])));
connect(ui.subButton, SIGNAL(clicked()), this, SLOT(getAndshowResult(znak[1])));

void GUI::getAndshowResult(char &f) {
    if (!getValues())
    {
        clearAllTextFields();
    }
    else {
        Kalkulator nowy(a, b);
        ui.wyswietlEdit->setText(QString::number(nowy.doCalc(f)));
    }
}