Best way to acces MainWindow instance from separate class

[scarecr0w132]

Hello,

What is the best way to access MainWindow instance from a separate class.

Main.cpp

...
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
 
    return a.exec();
}
...

...
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    
    return a.exec();
}
...

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How can I do this?

Thank you have a good day!