Circular dependencies with headers. Using #ifndef and #define - c++

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.

Answer 2

Do you use StartWindow in MainWindow? If not, simply remove the StartWindow.h include. Otherwise make the main_window a pointer instead of a variable.

Answer 3

In the file StartWindow.h remove #include "MainWindow.h" and add the forward declaration (before class StartWindow ...):
class MainWindow;
In the same file change the member MainWindow main_window to
const MainWindow* main_window;
or
const MainWindow& main_window;
In the latter case you would need to pass const MainWindow& in the constructor of StartWindow.