[mdmunoz]

You are a bit confused on the terminology. Cocoa is a framework, not a language. It's just a collection of classes and their methods which are available for the programmer to call. You can write Cocoa applications completely in C or C++ if you want to (though they will still only run on Macs). In fact, you can also use Java, Applescript, and, in Leopard, Ruby and Python.

You are correct, however, that the main language for Cocoa, Objective C (undoubtedly the language used in your book), isn't very popular at all outside of Mac development. The reasons for this are many, but the simplest one is that Obj-C and C++ are both regular C with object-oriented programming added in (and there are tradeoffs for either language). Since C++ has become so widespread, it leaves little room for another object-oriented C variant.

However, you would not be putting yourself at a disadvantage by learning Objective-C. Since it is basically C with a few enhancements, you will probably be learning all of C anyway if you will be doing serious development (in fact, Objective-C Cocoa programs will often have segments of pure C or even C++ code in them and work just fine). And switching to something like C++ from Obj-C should not be very difficult because of shared lineage with C.