From Jeremy Moles, "The version of GCC I use (4.4.3-4ubuntu5) gives the following warning

about the Observer header:

/home/cubicool/local/include/osg/Observer: In copy constructor
?osg::ObserverSet::ObserverSet(const osg::ObserverSet&)?:
/home/cubicool/local/include/osg/Observer:66: warning: base class ?class
osg::Referenced? should be explicitly initialized in the copy
constructor

I've been fixing this by hand by using the attached Observer header; it
does exactly what the warning requests. Purely cosmetic, I believe, but
other than that OSG seems to compile w/ -W -Wall just fine."

include/osg/Observer

@@ -63,7 +63,7 @@ class OSG_EXPORT ObserverSet : public osg::Referenced
 
     protected:
 
-        ObserverSet(const ObserverSet& rhs) {}
+        ObserverSet(const ObserverSet& rhs): osg::Referenced(rhs) {}
         ObserverSet& operator = (const ObserverSet& rhs) { return *this; }
         virtual ~ObserverSet();