25.2. Core Foundation

Core Foundation is a large library of C functions that are engineered around the concept of objects, which it terms opaque types or just types. These opaque types have an internal structure that is compatible with Objective-C objects, and a respectable number of Core Foundation types are interchangeable with Objective-C classes. This reciprocity is called the toll-free bridge. The goal of Core Foundation is to seamlessly provide a large number of operating system and framework services to both C and Objective-C programmers. C programmers use CFTypeRef values, which are pointers to opaque data structures, and pass them to Core Foundation functions. Objective-C programmers store object pointers and send them messages. The code executed is the same in both cases.

Most Core Foundation types do not have a toll-free bridge to Objective-C. If you need their functionality, you'll need to use the Core Foundation data types and functions directly. Core Foundation employs many of the same concepts as Objective-C, it just implements them with its own types, functions, and terminology. A comparison of common Core Foundation types and functions along with their Objective-C equivalents are listed in Table 25-1.