@ character isn’t used in C or C++ identifiers, so it’s used to introduce Objective-C language keywords in a way that won’t conflict with the other languages’ keywords. This enables the “Objective” part of the language to freely intermix with the C or C++ part.
Thus with very few exceptions, any time you see
@ in some Objective-C code, you’re looking at Objective-C constructs rather than C or C++ constructs.
The major exceptions are
Nil, which are generally treated as language keywords even though they may also have a
#define behind them. For example, the compiler actually does treat
id specially in terms of the pointer type conversion rules it applies to declarations, as well as to the decision of whether to generate GC write barriers.
Other exceptions are
bycopy; these are used as storage class annotations on method parameter and return types to make Distributed Objects more efficient. (They become part of the method signature available from the runtime, which DO can look at to determine how to best serialize a transaction.) There are also the attributes within
setter; those are only valid within the attribute section of a