Just a quick post to say that it's safe to delete the line
in b2WorldCallbacks.h line 30. It seems to be a forgotten leftover line still present in the current SVN revision.
I was trying to implement a b2ContactListener using an old documentation online and kept getting the error "use of undefined type 'b2ContactPoint'". Wherever I looked, I could only find this forward declaration and not the actual struct definition. I then realized my error and implemented the new version and everything is fine.
I know it's not a big deal, but since I stumbled upon it I though it would be nice to tell.