Runtime Versions and Platforms

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

In this section:

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 2.0 Runtime Reference.

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

In the legacy runtime, if you change the layout of instance variables in a class, you must recompile classes that inherit from it.
In the modern runtime, if you change the layout of instance variables in a class, you do not have to recompile classes that inherit from it.

In addition, the modern runtime supports instance variable synthesis for declared properties (see Declared Properties in The Objective-C 2.0 Programming Language).

Platforms

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

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

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