Question

I have the very simple following code:

main.cpp

#include "ui_library_browser.h"
#include <QtGui/QApplication>
#include "StartWindow.h"
int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  StartWindow w;
  w.show();
  return a.exec();
}

StartWindow.h

#ifndef STARTWINDOW_H_
#define STARTWINDOW_H_

#include <qwidget>
#include "MainWindow.h"

class StartWindow : public QWidget
{ 
  Q_OBJECT

public:
  StartWindow();
  ~StartWindow();
  MainWindow main_window;  //<-- Problem
};
#endif

MainWindow.h

#ifndef MAINWINDOW_H_
#define MAINWINDOW_H_

#include <qdialog.h>
#include "StartWindow.h"

class MainWindow : public QDialog
{
  Q_OBJECT

public:
  MainWindow();
  ~MainWindow();
};
#endif

This produces errors because of the inclusion of #include “StartWindow.h” in the MainWindow.h header. However, I thought the use of #ifndef and #define are to stop problems like this? Can someone clear this up for me?

Answer 1

So called “header guards” are used to prevent a bit different kind of error: including same header multiple time through different indirect inclusions in one compile unit. For example,
you include “a.h” from main.cpp and then include “b.h” from main.cpp, that includes “a.h” itself somewhere inside.

In your case two headers try to include each other circurally, that is not possible – C/C++ preprocessor works as simple text “copy-paste” and this case would invent infinite recursion of text insertion.

And I really don’t see why would you need “StartWindow.h” inclusion in “MainWindow.h” header.