SDI Applications and MainWindow

[r2com]

if you want to have a central place to store information about your app, you can invent whatever you want as a "document" class to hold that information.

In a nearby thread where I made a CTester class here http://www.qtcentre.org/threads/6644...w-and-my-class
does it mean that I "invented" the "SDI" concept there, so my CTester class is a "document", correct?

or you can make it a plain old C++ class with an instance owned by MainWindow. In that case, any "communication" is done through signals and slots you declare for MainWindow, and MainWindow reads and writes to the document instance through its C++ methods.

Again, this is how I implemented my CTester, right?

In that case, what I did there was a "tiny" minimal SDI application?

And another question is, what about the MDI then? Same thing? Except that I can instantiate my CTester tied to some control widget multiple times? Or any other notable differences?