C++ simple calculator in QT4 -

i trying create simple calculator in qt4. user inputs 2 numbers , there 4 command buttons calculations. (+ - * /). code is:

calculator.h

#ifndef calculator_h #define calculator_h  #include qwidget #include qgridlayout #include qlineedit #include qlabel #include qpushbutton #include qlcdnumber #include qstring #include qmessagebox #include qerrormessage  class calculator : public qwidget {  q_object  public:  //constructor  calculator();  public slots:  //function add 2 numbers user inputs  void addition();  //function subtract 2 numbers user inputs  void subtraction();  //function multiply 2 numbers user inputs  void multiply();  //function divide 2 numbers user inputs  void division();  //function clear widgets  void clearfields();  //function close window  void close();  private:  qlineedit* int1entry; qlineedit* int2entry; qlcdnumber* lineoutput; qerrormessage* error;  };  #endif 

calculator.cpp

#include "calculator.h"  //constructor  calculator::calculator() {  //{initializes , places widgets using layout      setwindowtitle("simple calculator");     qgridlayout* layout = new qgridlayout;     qpushbutton* additionbutton = new qpushbutton("+");     qpushbutton* subtractionbutton = new qpushbutton("-");     qpushbutton* multiplicationbutton = new qpushbutton("*");     qpushbutton* divisionbutton = new qpushbutton("/");      lineoutput = new qlcdnumber;      //result->setbinmode();      lineoutput->setsegmentstyle(qlcdnumber::flat);     lineoutput->setdigitcount(8);     qpushbutton* clear = new qpushbutton("clear");     qpushbutton* close = new qpushbutton("close");     layout->addwidget(int1entry, 2,1);     layout->addwidget(int2entry, 3,1);     layout->addwidget(additionbutton, 0,2);     layout->addwidget(subtractionbutton, 1,2);     layout->addwidget(multiplicationbutton, 2,2);     layout->addwidget(divisionbutton, 3,2);     layout->addwidget(new qlabel, 1, 0); //for spacing     layout->addwidget(lineoutput,3,0);     layout->addwidget(close, 4,1);     layout->addwidget(clear, 4,2);     setlayout(layout);      //}      //{connect signals , slots      connect(additionbutton, signal(clicked()), this, slot(addition()));     connect(subtractionbutton, signal(clicked()), this, slot(subtraction()));     connect(multiplicationbutton, signal(clicked()), this, slot(multiply()));     connect(divisionbutton, signal(clicked()), this, slot(division()));     connect(clear, signal(clicked()), this, slot(clearfields()));     connect(close, signal(clicked()), this, slot(close()));      //}  }  //function add 2 numbers  void calculator::addition() {      qstring strfisrt = int1entry->text();     qstring strsecond = int2entry->text();     bool ok;      int number1 = strfisrt.toint(&ok);     int number2 = strsecond.toint(&ok);     int result = number1 + number2;      qstring strresult;      strresult = strresult.number(result);     lineoutput->display(strresult); }  //function subtract 2 numbers  void calculator::subtraction()  {         qstring strfisrt = int1entry->text();         qstring strsecond = int2entry->text();         bool ok;          int number1 = strfisrt.toint(&ok);         int number2 = strsecond.toint(&ok);         int result = number1 - number2;         qstring strresult;         strresult = strresult.number(result);         lineoutput->display(strresult); }  //function multiply 2 numbers  void calculator::multiply()  {     qstring strfisrt = int1entry->text();     qstring strsecond = int2entry->text();          bool ok;          int number1 = strfisrt.toint(&ok);         int number2 = strsecond.toint(&ok);         int result = number1 * number2;          qstring strresult;          strresult = strresult.number(result);         lineoutput->display(strresult); }  //function divide 2 numbers  void calculator::division()  {     qstring strfisrt = int1entry->text();     qstring strsecond = int2entry->text();          bool ok;          int number1 = strfisrt.toint(&ok);         int number2 = strsecond.toint(&ok);         int result = number1 / number2;          qstring strresult;         strresult = strresult.number(result);         lineoutput->display(strresult); }  //clear widgets  void calculator::clearfields(){      int1entry->clear();     int2entry->clear();     lineoutput->display(0); }  //close window  void calculator::close(){      exit(0); } 

main.cpp

#include <qapplication> #include "calculator.h"  int main(int argc, char* argv[])  {      qapplication app(argc, argv);     calculator w;     w.show();     return app.exec(); } 

when run program blank ms-dos black screen. can u me please?

the problem never create instances of following items:

qlineedit* int1entry; qlineedit* int2entry; 

add following beginning of constructor:

int1entry = new qlineedit; int2entry = new qlineedit; 

you never assign instance of qerrormessage error either never use it, watch out later.