Runtime Versions and Platforms

There are different versions of the Objective-C runtime on different platforms.

Legacy and Modern Versions

There are two versions of the Objective-C runtime—“modern” and “legacy”. The modern version was introduced with Objective-C 2.0 and includes a number of new features. The programming interface for the legacy version of the runtime is described in Objective-C 1 Runtime Reference; the programming interface for the modern version of the runtime is described in Objective-C Runtime Reference.

The most notable new feature is that instance variables in the modern runtime are “non-fragile”:

In addition, the modern runtime supports instance variable synthesis for declared properties (see “Declared Properties” in The Objective-C Programming Language).

Platforms

iPhone applications and 64-bit programs on OS X v10.5 and later use the modern version of the runtime.

Other programs (32-bit programs on OS X desktop) use the legacy version of the runtime.