Objective-C is an Object-Oriented Programming language completely based on C, actually it's a thin Object-Oriented layer over C, whereas C++ is a language derived from C but that is now much different. In fact, you can use your whole C code and put it in Objective-C while it's not necessarily the case when putting the code directly in C++.
C++ and Objective-C appeared at the same time but they are slightly different, that's a new language to learn, however if you know C and/or C++, 3/4 of the learning is already made. The major differences reside in the syntax: How to write a method call (in Objective-C it's called a "message", we send messaged to objects), how to write a class, how to write methods, etc.
The difference that made Apple choose this language over C++ for their best API, is certainly because Objective-C is a completely dynamic language, all methods, compared to C++, are supposed "virtual", no non-virtual methods, the language is also completely typed dynamically, it's the runtime that takes care of method resolving and that also allows you to make message forwarding (if an object can't respond a message, it can redirect it to another object that might be able to answer to it).
All of that to say that Objective-C and C++ are not really compatible, you can use Objective-C++ which is a mix up of Objective-C and C++ but there's some restrictions in using it. If you want to develop for Macintosh, I advise you to learn Objective-C and Cocoa, than trying to stay with C++ and Carbon, Objective-C is not a difficult language, especially if you already know C or C++. And also a very pure object-oriented language.
If you want to learn how to program with Objective-C, there's the Apple course : [Object-Oriented Programming with Objective-C|http://developer.apple.com/documentation/Cocoa/Conceptual/OOP
ObjC/OOPObjC.pdf]
If you want to learn about Objective-C 2.0 (used in Leopard) there's the link about the whole programming language : [Introduction to The Objective-C 2.0 Programming Language|http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/O bjC.pdf]
And there's the tutorial to program a little application in Cocoa :
[Cocoa Application Tutorial|http://developer.apple.com/documentation/Cocoa/Conceptual/ObjCTutorial /ObjCTutorial.pdf]