Access UI Components from different source files

[cevou]

My UI is defined like this

mainwindow.h

#include <QMainWindow>
#include "ui_mainwindow.h"
#include "main_print_functions.h"
class MainWindow : public QMainWindow, public Ui::MainWindow {
    Q_OBJECT
 
public:
    MainWindow();
.
.
.
};

#include <QMainWindow>
#include "ui_mainwindow.h"
#include "main_print_functions.h"
class MainWindow : public QMainWindow, public Ui::MainWindow {
    Q_OBJECT

public:
    MainWindow();
.
.
.
};

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So how can I access the components in main_print_functions class?
This doesn't work:

MainWindow gui;
gui.checkBox->isChecked()

MainWindow gui;
gui.checkBox->isChecked()

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Can you post a piece of code that shows me how to do this?