With 10.5, Apple released a new version of Objective-C, dubbed Objective-C 2.0. While versions with earlier versions of OS X had added a few new features, they had retained binary-compatibility with the older versions. You can take a framework from OS X 10.0 and still use it on any newer version. Objective-C 2, however, provided a clean break.
With OS X 10.5, there are two runtime libraries for Objective-C, described by Apple as the “modern” and “legacy” runtime libraries. 64-bit code, and code compiled for the iPhone, will use the modern library, while 32-bit code uses the legacy library. All versions of OS X prior to 10.5 use the legacy library and only support 32-bit Cocoa applications.
Although the modern runtime is often called the Objective-C 2.0 runtime, this is not entirely true. Almost all of the features of Objective-C 2.0 can be used with the legacy runtime on Leopard. Some, such as fast enumeration, do not require any support from the runtime, but are limited to being useful on 10.5 since they require support from objects that is only provided in newer versions of Foundation. If you are writing 32-bit code, you can still use frameworks compiled on earlier versions of OS X with Objective-C 2, although you probably won’t be able to use the biggest new feature: garbage collection. The addition of garbage collection in
Objective-C 2 means that you don’t have to worry about memory management at all. This is an obvious advantage, although it comes with certain costs. The most obvious one is portability, since code using the garbage collector can only run on OS X 10.5 or newer, not on older versions of OS X and currently not on other platforms. In particular, this includes (current versions of) the iPhone. If you think you might want to port your code to the iPhone in the future, do not use garbage collection.
The other features are generally useful. Properties allow you to define a standard interface for accessing data on objects, and fast enumeration allows you to quickly get at all of the objects in a collection. Calling Objective-C 2.0 a new version is a little misleading, since properties and fast enumeration are really just syntactic sugar and garbage collection is optional. The additions added in 10.3 for structured exception handling were, arguably, as significant a set of changes and did not warrant a new version number.
Source of Information : Addison Wesley - Cocoa Programming Developers Handbook (December 2009)
With OS X 10.5, there are two runtime libraries for Objective-C, described by Apple as the “modern” and “legacy” runtime libraries. 64-bit code, and code compiled for the iPhone, will use the modern library, while 32-bit code uses the legacy library. All versions of OS X prior to 10.5 use the legacy library and only support 32-bit Cocoa applications.
Although the modern runtime is often called the Objective-C 2.0 runtime, this is not entirely true. Almost all of the features of Objective-C 2.0 can be used with the legacy runtime on Leopard. Some, such as fast enumeration, do not require any support from the runtime, but are limited to being useful on 10.5 since they require support from objects that is only provided in newer versions of Foundation. If you are writing 32-bit code, you can still use frameworks compiled on earlier versions of OS X with Objective-C 2, although you probably won’t be able to use the biggest new feature: garbage collection. The addition of garbage collection in
Objective-C 2 means that you don’t have to worry about memory management at all. This is an obvious advantage, although it comes with certain costs. The most obvious one is portability, since code using the garbage collector can only run on OS X 10.5 or newer, not on older versions of OS X and currently not on other platforms. In particular, this includes (current versions of) the iPhone. If you think you might want to port your code to the iPhone in the future, do not use garbage collection.
The other features are generally useful. Properties allow you to define a standard interface for accessing data on objects, and fast enumeration allows you to quickly get at all of the objects in a collection. Calling Objective-C 2.0 a new version is a little misleading, since properties and fast enumeration are really just syntactic sugar and garbage collection is optional. The additions added in 10.3 for structured exception handling were, arguably, as significant a set of changes and did not warrant a new version number.
Source of Information : Addison Wesley - Cocoa Programming Developers Handbook (December 2009)
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