Use class A which includes class B from class B - c++

Question

I have four files:
mainwindow.h
#pragma once // MAINWINDOW_H
#include <QMainWindow>
#include <QApplication>
#include "maincontent.h"
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = nullptr);
~MainWindow();
void setStatusBarMessage(QString message);
signals:
public slots:
void exit();
private:
void setMenuBar();
MainContent* content;
};
maincontent.h
#pragma once // MAINCONTENT_H
#include "statistic.h"
#include "information.h"
#include <QWidget>
#include <QHBoxLayout>
class MainContent : public QWidget
{
Q_OBJECT
public:
explicit MainContent(QWidget *parent = nullptr);
signals:
public slots:
private:
QHBoxLayout* layout;
Statistic* statistic;
Information* financFlow;
};
information.h
#pragma once // INFORMATION_H
#include <QPushButton>
#include <QWidget>
//#include "mainwindow.h" //error
class Information : public QWidget
{
Q_OBJECT
public:
explicit Information(QWidget *parent = nullptr);
signals:
public slots:
private:
QPushButton* button;
};
statistic.h
#pragma once // STATISTIC_H
#include <QWidget>
#include <QHBoxLayout>
#include <QListView>
class Statistic : public QWidget
{
Q_OBJECT
public:
explicit Statistic(QWidget *parent = nullptr);
signals:
public slots:
private:
QListView* listView;
};
Now I will use the MainWindow::setStatusBarMessagemethod from the Information class.
But when I include the MainWindow in the Information class: #include "mainwindow.h"
I become the error: MainContent does not name a type in line 22 in mainwindow.h
First I don't know why the compiler can't find MainContent bacause in MainWindow I included the "maincontent.h", does the preprocessor only include "mainwindow.h" but not the "maincontent.h" in the "mainwindow.h"?
I see that with #include "mainwindow.h" a recursion arise but that shouldn't be a problem because of #pragma once or?
Next I tried to include the "mainwindow.h" in the information.cpp file but then I have the problem that I everytime give the MainWindow object by parameter and can't hold a MainWindow in my class
My main problem is that the MainWindow has a statusBar Object and I will set the statusBar message from everywhere. How can I do this, exist a Pattern or someting for that?
How can I solve this Problem or where I make a thinking mistake?
Thanks for your help.

Answer 1

You should use forward declaration for your included classes. Since pointers itself do not need to know a fully declared class (pointer size is always the same) you can easily get around this:
In header File instead of include the class just declare it:
#include <QMainWindow>
#include <QApplication>
//#include "maincontent.h" // REmove these
class MainContent;
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = nullptr);
~MainWindow();
void setStatusBarMessage(QString message);
signals:
public slots:
void exit();
private:
void setMenuBar();
MainContent* content;
};
In cpp file now include the header again. Do this for all includes only having a pointer to given class.
As mentioned by #dempzorz removing the circular dependency is always prefered, but not always possible.

Answer 2

You have a circular dependency. You can't have information.h include mainwindow.h and also have mainwindow.h include information.h. You should design dependencies in a hierarchy, where items lower in the tree do not include items higher in the tree.
You should have a look at this link to maybe give you a better understanding of how to structure your objects:
https://en.wikipedia.org/wiki/Circular_dependency