calculatorQT

//main.cpp
#include "mainwindow.h"
#include "calculator.h"
#include <QApplication>

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

    calculator.setWindowTitle("Calculator");
    calculator.resize(230, 200);

    calculator.show();
    return app.exec();
}

//calculator.h
#ifndef CALCULATOR_H
#define CALCULATOR_H

#include <QPushButton>
#include <QBoxLayout>
#include <QWidget>
#include <QStack>

class QLCDNumber;
class QPushButton;

//===============================================================

class Calculator : public QWidget {
    Q_OBJECT
private:
    QLCDNumber*         m_plcd;
    QStack<QString>     m_stk;
    QString             m_strDisplay;

public:
    Calculator(QWidget* pwgt = 0);

    QPushButton* createButton(const QString& str);
    void calculate(     );

public slots:
    void slotButtonClicked();
};
#endif // CALCULATOR_H

//calculator.cpp

#include "calculator.h"
#include <QLCDNumber>
Calculator::Calculator(QWidget *pwgt/*= 0*/) : QWidget(pwgt) {
    m_plcd = new QLCDNumber(12);
    m_plcd->setSegmentStyle(QLCDNumber::Flat);
    m_plcd->setMinimumSize(150, 50);

    QChar aButtons[4][4] = {{'7', '8', '9', '/'},
                            {'4', '5', '6', '*'},
                            {'1', '2', '3', '-'},
                            {'0', '.', '=', '+'}
                           };
    //Layout setup
    QGridLayout *ptopLayout = new QGridLayout;
    ptopLayout->addWidget(m_plcd, 0, 0, 1, 4);
    ptopLayout->addWidget(createButton("CE"), 1, 3);

    for(int i = 0; i < 4; ++i) {
        for(int j = 0; j < 4; ++j) {
            ptopLayout->addWidget(createButton(aButtons[i][j]), i+2, j);
        }
    }

    setLayout(ptopLayout);
}

QPushButton* Calculator::createButton(const QString &str) {
    QPushButton* pcmd = new QPushButton(str);
    pcmd->setMinimumSize(40, 40);
    connect(pcmd, SIGNAL(clicked()), SLOT(slotButtonClicked()));
    return pcmd;
}

void Calculator::calculate() {
    double          dOperand2      =   m_stk.pop().toDouble();
    QString         strOperation   =   m_stk.pop();
    double          dOperand1      =   m_stk.pop().toDouble();
    double          dResult        =   0;

    if(strOperation == "+") {
        dResult = dOperand1 + dOperand2;
    }
    if(strOperation == "-") {
        dResult = dOperand1 - dOperand2;
    }
    if(strOperation == "/") {
        dResult = dOperand1 / dOperand2;
    }
    if(strOperation == "*") {
        dResult = dOperand1 * dOperand2;
    }

    m_plcd->display(dResult);
}

void Calculator::slotButtonClicked() {
    QString str = ((QPushButton*)sender())->text();

    if(str == "CE") {
        m_stk.clear();
        m_strDisplay = "";
        m_plcd->display("0");
        return;
    }
    if(str.contains(QRegExp("[0-9]"))) {
        m_strDisplay += str;
        m_plcd->display(m_strDisplay.toDouble());
    }
    else if(str == ".") {
        m_strDisplay += str;
        m_plcd->display(m_strDisplay);
    }
    else {
        if(m_stk.count() >= 2) {
            m_stk.push(QString().setNum(m_plcd->value()));
            calculate();
            m_stk.clear();
            m_stk.push(QString().setNum(m_plcd->value()));
            if(str != "=") {
                m_stk.push(str);
            }
        }
        else {
            m_stk.push(QString::number(m_plcd->value()));
            m_stk.push(str);
            m_strDisplay = "";
        }
    }
}