I'd like to add a GUI to one of my C++ codes. As the software should run under Linux
and Windows I think about using QT. A special requirement is that I'd like to
visualize 3D objects (triangular meshes). It should be possible to turn and zoom
the objects, and it should be possible to select a certain triangle in order to
get information about its corners.

Is there something like this included in QT or is there a better library for this

Thank you,
best regards

Well. Qt has fairly straight-forward OpenGL support through the QGLWidget, for example this simple NeHe tutorials in Qt. For all the pan/rotate/zoom and select operations, I'm not sure if there are much off-the-shelf components or actions that can do this. Fortunately, this isn't all that hard to implement with OpenGL transforms and the basic mouse-events from Qt.

If you want a more complete tool, I have heard that VTK is very feature-rich, but I have never used it.