Dan,
The X11 window ID was an example. Does Qt have an analogous widget ID that is something different than X11 window ID?
Dan
Difficult to answer, I think it's a wrong question :) X11 is a window system, Qt is a C++ application framework. Qt is object-oriented, so every unique QWidget object has a memory address (which identifies the object), so I would say that is what the ID is. In fact, the memory address is used to embed QWidgets in other QWidgets. I guess a Qt application is not primarily intended to be "swallowed" by another application or to expose its internals.
Embedding one application's widget into another application's container is done through the XEmbed protocol under X11. Qt provides container and widget classes implementing the XEmbed protocol (QX11EmbedWidget and QX11EmbedContainer). This, however, is a X11-only solution (though equivalent exists under Windows, I don't know about OS X).
Michael.