Yes, you are right, aamer4yu. Thank you

Now it works with the following code:

Qt Code: 
  1. #include <QApplication>
  2. #include <QPushButton>
  3. #include <QMessageBox>
  4. #include <mainframe.h>
  5. #include "dialog_start.h"
  6. #include "OscillationGRN.h"
  7. #include <iostream>
  8.  
  9. int main(int argc, char **argv)
  10. {
  11.  
  12.      QApplication app(argc, argv);
  13.  
  14.      // first appears a popup, which will offer to the user 2 different kinds of problems
  15. 		 QMessageBox msgBox;
  16. 		 msgBox.setIcon(QMessageBox::Question);
  17. 		 msgBox.setText("Which one of the optimization problems do you want to execute?");
  18. 		 msgBox.setInformativeText("Make your choice by clicking on button.");
  19. 		 QPushButton *OscillationButton = msgBox.addButton("Sustained Oscillation", QMessageBox::ActionRole);
  20. 		 QPushButton *BenchmarkButton = msgBox.addButton("Constrained Problems",QMessageBox::NoRole);
  21.  
  22. 		 msgBox.exec();
  23.  
  24. 		  mainFrame mainWind;		// build Main Window
  25.  
  26. 		  dialogStart start(0, &mainWind);  // build Initialization window
  27.  
  28. 		  OscillationGRN evoGRN;	// build OscillationGRN window
  29.  
  30.      if(msgBox.clickedButton() == BenchmarkButton){	// if the user decide to cope with optimization of constrained benchmark problems
  31.  
  32.       	start.show();		// the mainWindow will be opened after the user click on START button within dialogStart
  33.  
  34.      }
  35.      else if(msgBox.clickedButton() == OscillationButton){		// if the user decide to cope with generation of oscillation by means of Gene Regulatory Networks
  36.  
  37.     	 evoGRN.show();
  38.  
  39.      }
  40.      return app.exec();
  41.  
  42. 	 exit(EXIT_SUCCESS);
  43.  
  44. }
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