Include header main window to a child window in QT - c++

Question

I try to include header window into his child window. Let see a code:
//header of main window
#include "second_window.h" // include child window
class mainclass : public QMainWindow
{
Q_OBJECT
public:
mainclass(QWidget *parent = 0);
~mainclass();
second_window *h_secondwindow = nullptr;
private:
Ui::mainclass ui;
};
And Second
//header second_window.h
#include "mainwindow.h" // only this from mainwindow class
class second_window : public QWidget
{
Q_OBJECT
public:
third_window * h_third_window = nullptr;
second_window(QWidget *parent = 0);
~second_window();
private:
void reDrawTable();
Ui::second_window ui;
};
#endif // second_window_H
And when in "second_window.h" i try to include "header of main window" (lets call it mainwindow.h )
I got error by this line (in main window) :
second_window *h_secondwindow = nullptr;
Errors like:
//qdatetime.h ( i dont edit this file ;0 )
error C2059: syntax error : '::'
static inline qint64 nullJd() { return std::numeric_limits<qint64>::min(); }
//mainwindow.h
error C2143: syntax error : missing ';' before '*'
second_window *h_secondwindow = nullptr;
I need this variable bcoz i have to operate in main window method on this child window.
Any idea?

Answer 1

Try, cross checking your "second_window.h" to see if there any code errors, it might be there are some few errors. Such errors mainly happen with pointers and references, so an error might be possible!

Answer 2

You shouldn't include the header files inside each other. You can use a forward declaration of mainwindow class in second_window.h :
//header second_window.h
class mainwindow;
Forward declarations mean that you can only use that class as a pointer in the header file and not a class instance. So using this way you can now have a pointer to your mainwindow in second_window.h.
But it is recommended to check if you can change your design and eliminate this circular dependency.

Answer 3

I think your code is dark means.
Why does second_window include main_window?
You should init in main window.h:
second_window* _secondWindowUi;
in .cpp file :
_secondWindowUI = new second_window();
It is easy!