Object-related syntax in ObjC is completely alien to C. Object-related syntax in C++ (mostly) extends C structure syntax.
Yes, ObjC takes its cues from Smalltalk. C++ does not. And so... ?
[EDIT: ok, so people want to interpret "superset" as meaning "every valid C program is a valid Objective C program too. This is, with very few exceptions, true of C++ as well ]