Accessing Outside of a class - c++

Question

I am trying to access a QgraphicsView's Scene outside of a class. I can normally do this when I create a class that is derived from QGraohicsView, but this class is the MainWindow which derives from QMainWIndow and I can have it extend QGraphicsView because there is a conflict when you call the .show() method as the compiler does not know which one to choose.
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{}
So I create a new instance of QgraphicsView and make it public and also Qgraphicsscene and make it public.
Then in the exterior class
extern MainWindow * mainwindow
But when I try to access it and I get a undefined reference error.
mainwindow->view->scene()->addItem(item); or
mainwindow->mainwindow.scene->addItem(item);
Neither of them work.
I know its breaking encapsulation but there is no other way around this in this particular case.
** What I am trying to do is access a QgraphicsView's Scene outside of its class?
** MainWindow has public variables
QGraphicsScene *scene;
QGraphicsView * view;
In MainWindow.cpp
scene = new QGraphicsScene(this);
view = new QGraphicsView(scene);
view.show();

Answer 1

This works for me. Since view in your case was a pointer, you must access its members via ->, not .. But all that is superfluous anyway - you should store everything by value as much as possible and let the compiler worry about making sure the resources are freed when no longer needed. That's why you're using C++, not C, after all.
// https://github.com/KubaO/stackoverflown/tree/master/questions/simple-view-33508582
#include <QtWidgets>
class MainWindow : public QMainWindow {
Q_OBJECT
QGraphicsScene m_scene;
QWidget m_central;
QGraphicsView m_view; // must be declared after m_central per C++ semantics
QGridLayout m_layout;
public:
MainWindow(QWidget * parent = 0) :
QMainWindow(parent),
m_layout(&m_central) {
setCentralWidget(&m_central);
m_layout.addWidget(&m_view, 0, 0);
m_view.setScene(&m_scene);
}
QGraphicsScene * scene() { return &m_scene; }
QGraphicsView * view() { return &m_view; }
};
int main(int argc, char ** argv) {
QApplication app(argc, argv);
MainWindow win;
win.scene()->addEllipse(0, 0, 10, 10);
win.show();
return app.exec();
}
#include "main.moc"
You should also decide whether you need a QMainWindow at all. Just because a Qt Creator template uses it, doesn't mean you should blindly use it too. If you're not using QMainWindow's docking area functionality, a QDialog would be a more sensible base class to use:
// https://github.com/KubaO/stackoverflown/tree/master/questions/simple-view-33508582
#include <QtWidgets>
class MainWindow : public QDialog {
Q_OBJECT
QGraphicsScene m_scene;
QGraphicsView m_view;
QGridLayout m_layout;
public:
MainWindow(QWidget * parent = 0) :
QDialog(parent),
m_layout(this) {
m_layout.addWidget(&m_view, 0, 0);
m_view.setScene(&m_scene);
}
QGraphicsScene * scene() { return &m_scene; }
QGraphicsView * view() { return &m_view; }
};
int main(int argc, char ** argv) {
QApplication app(argc, argv);
MainWindow win;
win.scene()->addEllipse(0, 0, 10, 10);
win.show();
return app.exec();
}
#include "main.moc"