main.cpp; myCalc.cpp, myCalc.h

/*------------------------------------------*/
/* this is the my main.cpp file */

#include <QApplication>
#include "qtCalc.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    Calc *dialog = new Calc();
    dialog­>show();
    return app.exec();
}

/*------------------------------------------*/
/* this is th my myCalc.cpp file */

#include <QtGui>
#include "myCalc.h"

Calc::Calc( QWidget *parent )
    : str( "" ), opCod( '0' )
{
    setupUi(this); // this sets up GUI

// signals/slots mechanism in action
    connect(pushButton_CE,SIGNAL(clicked()),this,SLOT(clear()));
    connect(pushButton_Igual,SIGNAL(clicked()),this,SLOT(igualClick()));
    connect(pushButton_0,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_1,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_2,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_3,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_4,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_5,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_6,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_7,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_8,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_9,SIGNAL(clicked()),this,SLOT(digitClick()));
    connect(pushButton_Soma,SIGNAL(clicked()),this,SLOT(operationClick()));
    connect(pushButton_Sub,SIGNAL(clicked()),this,SLOT(operationClick()));
    connect(pushButton_Div,SIGNAL(clicked()),this,SLOT(operationClick()));
    connect(pushButton_Mult,SIGNAL(clicked()),this,SLOT(operationClick()));
    connect(pushButton_Exp,SIGNAL(clicked()),this,SLOT(operationClick()));
    connect(pushButton_Mod,SIGNAL(clicked()),this,SLOT(operationClick()));

    void Calc::clear()
    {
        resultArea­>clear();
        resultArea­>setText( "0" );
        str = "";
        opCod = '0';
    }

    void Calc::digitClick()
    {
        QPushButton *clickedButton = qobject_cast<QPushButton*>(sender());
        if ( clickedButton == pushButton_0 && str == "" )
            ;
        else
           str += clickedButton­>text().toStdString();

        resultArea­>setText( QString::fromStdString(str));
    }

    void Calc::operationClick()
    {
        QPushButton *clickedButton = qobject_cast<QPushButton *>(sender());
        if(opCod == '0')
        {
            string res = clickedButton­>text().toStdString();
            opCod = res[0];
            istringstream temp( str );
            temp >> op1;
            str = "";
        }
    }

    void Calc::igualClick()
    {
        if (str.length() > 256)
            resultArea­>setText("Caracter limit exceded");
        else
        {
            istringstream temp ( str );
            temp >> op2;
            QString res;
            bool valid = true;
            switch(opCod)
        {
            case '+':
                res.setNum( op1 + op2 );
                break;
            case '­':
               res.setNum( op1 + op2 );
               break;
            case '*':
               res.setNum( op1 * op2 );
               break;
            case '/':
               res.setNum( op1 / op2 );
               break;
            case '^':
               res.setNum( pow(op1, op2) );
               break;
            case '%':
               res.setNum(op1 & op2);
               break;
               default:
               valid = false;
          }
          if(!valid)
              resultArea­>setText("Erro: Invalid Operator.");
          else
              resultArea­>setText( res );
              str = "";
              opCod = '0';
    }
}

/*------------------------------------------*/
/* this is the my myCalc.h file */

#ifndef QTCALC_H
#define QTCALC_H
#include "ui_myCalc.h"
#include <string>
using std::string;
class Calc : public QWidget, private Ui::qtCalc
{
    Q_OBJECT
    public: Calc( QWidget *parent = 0);
    public slots:
        void clear();
        void igualClick();
        void digitClick();
        void operationClick();
    private:
        string str;
        char opCod;
        int op1,op2;
};
#endif