QObject::connect question

I was reading a beginners tutor on a push button that closes then a user clicks it. in qt creator I created a new qt gui application using the default file names. I then went to the form designer and put on the main window a push button.

in the mainwindow.cpp, I created the following code to put into the push button click function. The problem is that in the code &a, the “a” is defined in the main.cpp file only. I would like to close the application from the mainwindow.cpp instead but i don’t know how or even if that is possible. I would like to close it in the mainwindow.cpp because that way i could do my from design in the designer, select my signal and then do the code all within the mainwindow.cpp. is that possible to do?

here is the mainwindow.cpp code.