From Laurens Voerman,

"E:\osg\osgSvnGit\OpenSceneGraph\include\osg/Callback(286): warning C4099: 'osg::DrawableUpdateCallback' : type name first seen using 'class' now seen using 'struct' (E:\osg\osgSvnGit\OpenSceneGraph\src\osgUtil\RenderBin.cpp)
          E:\osg\osgSvnGit\OpenSceneGraph\include\osg/Callback(27) : see declaration of 'osg::DrawableUpdateCallback'

attached is a modified version of include/osg/Callback:
changing
- struct OSG_EXPORT DrawableUpdateCallback : public virtual Callback
- {
to
+ class OSG_EXPORT DrawableUpdateCallback : public virtual Callback
+ {
+ public:

and the same changes for DrawableEventCallback and DrawableCullCallback"

include/osg/Callback

@@ -283,8 +283,9 @@ class Drawable;
 class State;
 class RenderInfo;
 
-struct OSG_EXPORT DrawableUpdateCallback : public virtual Callback
+class OSG_EXPORT DrawableUpdateCallback : public virtual Callback
 {
+public:
     DrawableUpdateCallback() {}
 
     DrawableUpdateCallback(const DrawableUpdateCallback&,const CopyOp&) {}
@@ -302,8 +303,9 @@ struct OSG_EXPORT DrawableUpdateCallback : public virtual Callback
 };
 
 
-struct OSG_EXPORT DrawableEventCallback : public virtual Callback
+class OSG_EXPORT DrawableEventCallback : public virtual Callback
 {
+public:
     DrawableEventCallback() {}
 
     DrawableEventCallback(const DrawableEventCallback&,const CopyOp&) {}
@@ -317,8 +319,9 @@ struct OSG_EXPORT DrawableEventCallback : public virtual Callback
     virtual void event(osg::NodeVisitor*, osg::Drawable*) {}
 };
 
-struct OSG_EXPORT DrawableCullCallback : public virtual Callback
+class OSG_EXPORT DrawableCullCallback : public virtual Callback
 {
+public:
     DrawableCullCallback() {}
 
     DrawableCullCallback(const DrawableCullCallback&,const CopyOp&) {}