MainWindow.h
Qt Code: 
  1. #include <QMainWindow>
  2.  
  3. class MainWindow : public QMainWindow
  4. {
  5.     Q_OBJECT
  6. public:
  7.     explicit MainWindow(QWidget *parent = 0);
  8. };
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MainWindow.cpp
Qt Code: 
  1. #include "MainWindow.h"
  2.  
  3. MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
  4. {
  5. #ifdef QT_DEBUG
  6.     setWindowTitle("DEBUG");
  7. #else
  8.     setWindowTitle("RELEASE");
  9. #endif
  10.  
  11. }
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Application.h
Qt Code: 
  1. #include <QApplication>
  2.  
  3. class MainWindow;
  4.  
  5. class Application : public QApplication
  6. {
  7. public:
  8.     Application(int argc, char** argv);
  9.     ~Application();
  10.     MainWindow* mainWindow() const;
  11. private:
  12.     MainWindow* _mainWindow;
  13. };
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Application.cpp
Qt Code: 
  1. #include "Application.h"
  2. #include "MainWindow.h"
  3.  
  4. Application::Application(int argc, char** argv) : QApplication(argc, argv)
  5. {
  6.     _mainWindow = new MainWindow;
  7. }
  8.  
  9. Application::~Application(){
  10.     delete _mainWindow;
  11.     _mainWindow = 0;
  12. }
  13.  
  14. MainWindow * Application::mainWindow() const
  15. {
  16.     return _mainWindow;
  17. }
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main.cpp
Qt Code: 
  1. #include "Application.h"
  2. #include "MainWindow.h"
  3.  
  4. int  main(int argc, char** argv)
  5. {
  6.     Application app(argc, argv);
  7.     app.mainWindow()->show();
  8.     return app.exec();
  9. }
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The code above run perfectly fine under release build, but will cause SIGSEGV Error everytime under debug build.
My system is Ubuntu 11.04 64-bits, Qt 4.8/4.7.4