Thanks again for the reply. My problem is that the interface class is used when compiling the plugin library as well as compiling the main application. In both the return type of loadViewer() need to be specified. For the plugin it makes sense to use the return type of matrixReal as shown in my previous posts.

However for the main program, it can't use matrixReal because it does not know anything about it. This is what I don't understand... Should I use QWidget instead in the interface used to build the main program? I would think that the interface classes should be exactly the same.