basically, what I'm trying to do is define an interface that a bunch of plugins should follow. Each plugin should implement a series of slots. If they fail to do so, they need to produce a compile-time error. Theoretically, I could do this by not inheritting the base class at all and implementing the slots in the individual plugins, but if a plugin fails to implement a slot, it will still compile and run with weird results.

Here's the structure of what I've got:
Qt Code: 
  1. .
  2.         +------------------+         +--------------------------------+
  3.         | Plugin Interface |         | Abstract base class with slots |
  4.         +------------------+         +--------------------------------+
  5.                  |                            |
  6.         +------------------+         +------------------------------+
  7.         | Plugins (a,b...) |---------| QWidget/QGLWidget sub-class  |
  8.         +------------------+         +------------------------------+
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The section on the right is a simplified version of my previous diagram. My plugin has a "has a" relationship with the QWidget / QGLWidget class, which it would return on request (effectively bypassing the plugin itself requiring QWidget, and theoretically allowing the QWidget to have abstract slots defined that all plugins should follow).