Qt Code: 
  1. #include <QApplication>
  2. #include <QPushButton>
  3. #include <QLabel>
  4. #include <QSpinBox>
  5. #include <QLCDNumber>
  6. #include <QHBoxLayout>
  7. #include <QVBoxLayout>
  8.  
  9. class Calculator : public QWidget {
  10.     Q_OBJECT
  11. public:
  12.   Calculator(QWidget *parent = 0) : QWidget(parent){
  13.     minus = new QLabel("-");
  14.     equal = new QPushButton("&Execute");
  15.     num1 = new QSpinBox;
  16.     num2 = new QSpinBox;
  17.     answer = new QLCDNumber(3);
  18.  
  19.     QHBoxLayout *layout = new QHBoxLayout;
  20.     layout->addWidget(num1);
  21.     layout->addWidget(minus);
  22.     layout->addWidget(num2);
  23.     layout->addWidget(answer);
  24.     QHBoxLayout *buttons = new QHBoxLayout;
  25.     buttons->addStretch();
  26.     buttons->addWidget(equal);
  27.     buttons->addStretch();
  28.     QVBoxLayout *mainLayout = new QVBoxLayout(this);
  29.     mainLayout->addLayout(layout);
  30.     mainLayout->addLayout(buttons);
  31.     QObject::connect(equal, SIGNAL(clicked()), answer, SLOT(calculate()));
  32.       }
  33. public slots:
  34.   void calculate() {
  35.     answer->display(num1->value()-num2->value());
  36.     disconnect();
  37.   }
  38. private:
  39.   QLabel *minus;
  40.   QPushButton *equal;
  41.   QPushButton *help;
  42.   QSpinBox *num1;
  43.   QSpinBox *num2;
  44.   QLCDNumber *answer;
  45. };
  46.  
  47. int main(int argc, char **argv){
  48.   QApplication app(argc, argv);
  49.   Calculator w;
  50.   w.show();
  51.   return app.exec();
  52. }
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When the compiler is linking to the .exe it gives me this error.
Qt Code: 
  1. C:\Program Files\CodeBlocks\MinGW\bin\..\lib\gcc\mingw32\3.4.5\..\..\..\..\include\c++\3.4.5\bits\stl_algobase.h:(.text$_ZN10CalculatorD1Ev[Calculator::~Calculator()]+0xb)||undefined reference to `vtable for Calculator'|
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Thank you in advance.