They're two different languages.
As it happens, they were both invented around the same time (early 1980's), both derived from C, and they both tried to achieve "object orientation". Although each had radically different interpretations as to what "object oriented" actually meant...
... and, IMHO, the software world would have been a much happier place had Objective C prevailed instead of C++
If you're serious about the job, I would try to get into an "Objective-C frame of mind".
"C++ vs. Objective-C" is very much:
a) PC vs. Mac
b) MFC vs. Cocoa
c) Clumsy vs. Elegant
d) Oil vs. Water
Again ... IMHO