Change object of other class qt - c++

Question

I am using Qt and C++, after click on menu item second window appears, and after click on a button in second menu (slot saveData()), I want to change object (obj_map) of class MainMenu. Is it possible, and how to do it the best way? Because I now cannot modify obj_map, because it is in different class. I tried to do something with pointers, but the result was a segmentation fault.
Main Window:
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
map obj_map;
public Q_SLOTS:
void saveMap();
private:
Ui::MainWindow *ui;
};
Other window which appears after click in on menu item in main window.
namespace Ui
{
class PreferencesWindow;
}
class PreferencesWindow : public QWidget
{
Q_OBJECT
public:
explicit PreferencesWindow(QWidget *parent = 0);
public Q_SLOTS:
void saveData();
private:
Ui::PreferencesWindow *uip;
};
From here I need to change obj_map
void PreferencesWindow::saveData()
{
// FROM HERE I NEED TO CHANGE obj_map
}
Preferences object is created in a slot:
void MainWindow::saveMap()
{
PreferencesWindow *p = new PreferencesWindow();
p->show();
}

Answer 1

You could use signals and slots: when saveData() is called, emit a signal, like emit saveDataClicked() and catch that signal in the MainWindow with a slot called change_obj_map. There, you can do your changes.
So, in MainWindow you can write:
connect (PreferencesWindow, SIGNAL(saveDataClicked()), this, SLOT(change_obj_map());
and then in the slot:
void change_obj_map()
{
// Do your changes here
}
Another way is having a local obj_map in PreferencesWindow that is a pointer to the address of obj_map in MainWindow. So, when you create PreferencesWindow, you can just pass the address of MainWindow's obj_map to the constructor and assign that address to the local variable obj_map.

Answer 2

As PreferencesWindow objects are created by MainWindow, the easiest is to have PreferencesWindow objects store a pointer to MainWindow:
class MainWindow;
class PreferencesWindow : public QWidget
{
Q_OBJECT
public:
explicit PreferencesWindow(MainWindow *parent = 0);
public Q_SLOTS:
void saveData();
private:
Ui::PreferencesWindow *uip;
MainWindow* m_mainwindow;
};
Pass the pointer upon construction:
void MainWindow::saveMap()
{
PreferencesWindow *p = new PreferencesWindow( this );
p->show();
}
Then use it:
PreferencesWindow::PreferencesWindow(MainWindow *parent) :
QWidget( parent ),
m_mainwindow( parent )
{
}
void PreferencesWindow::saveData()
{
// FROM HERE I NEED TO CHANGE obj_map
m_mainwindow->obj_map.....it's accessible!
}