Runtime error when run the application, and the reason is the Qlable

Question

The problem is when run the application, appears a message to close the application without clarifying the cause of the problem.

the application is a simple calculator in order to addition two numbers.
This application contains six GUI objects.
Two QSpinBox to input the numbers.
Three Qlabel, two Qlabel to display +, = ,and one other to output the result of addition of two number, and this object is the reason of the problem.
Finally, one QPushButton to display the result in a Qlabel.

Now, It's time to display code:
I have three files(main.cpp, calculator.h, calculator.cpp).

-- Main.cpp --

#include "calculat.h"

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

   Calculator calc;
   calc.show();

   return app.exec();
}

-- calculator.h --

#ifndef CALCULATOR_H
#define CALCULATOR_H

#include <QWidget>

class QSpinBox;
class QLabel;

class Calculator : public QWidget {
   Q_OBJECT
public:
   Calculator();

private slots:
   void on_addNumber_clicked();

public:
   QSpinBox *firstValueSpinBox;
   QSpinBox *secondValueSpinBox;
   QLabel *resultLabel;
};

#endif // CALCULATOR_H

-- calculator.cpp --

#include "calculator.h"
#include <QPushButton>
#include <QSpinBox>
#include <QLabel>
#include <QHBoxLayout>

Calculator::Calculator(){
   QPushButton *addButton = new QPushButton("Add");
   firstValueSpinBox = new QSpinBox();
   secondValueSpinBox = new QSpinBox();
   resultLabel = new QLabel();
   QLabel *addLabel = new QLabel("+");
   QLabel *equalLabel = new QLabel("=");

   connect(addButton, SIGNAL(clicked()), this, SLOT(on_addNumber_clicked()));

   QHBoxLayout *layout = new QHBoxLayout(this);
   layout->addWidget(firstValueSpinBox);
   layout->addWidget(addLabel);
   layout->addWidget(secondValueSpinBox);
   layout->addWidget(addButton);
   layout->addWidget(equalLabel);
   layout->addWidget(resultLabel);
}

void Calculator::on_addNumber_clicked(){
   int num = this->firstValueSpinBox->value();
   int num2 = this->secondValueSpinBox->value();
   QString outResult = QString::number(num + num2);
   resultLabel->setText(outResult);       //<< the problem here
}

I doubt in this line:

resultLabel->setText(outResult);

When remove that previous line, the application work fine.
Conclusion, the problem in this Qlabel object that responsible for display the final result.

QLabel *resultLabel; // declaration in calculator.h

resultLabel->setText(outResult); // in calculator.cpp

Answer 1

There are no crashing bugs in your code. It runs just fine. Your problem is a rather classical result of stale object files that don't match the code anymore. The code generated from moc_calculator.cpp is stale. How are you building the project: manually, or using make/qmake? If you use make/qmake or make/cmake (say, from Qt Creator), do the following:

1. Completely remove the build directory (you'll find it one directory above the sources).

2. Rebuild.

There's a functional bug that doesn't result in a crash, simply in a misbehavior. Maybe it's even a typo. Instead of resultLabel->setText("outResult");, you want

resultLabel->setText(outResult);