Memory Management
One feature of the Cocoa environment is its facility for managing dynamically allocated memory. Cocoa's NSObject class, from which most classes, both vendor and user, are derived, implements a reference counting scheme for memory management. Objects derived from the NSObject root class respond to a retain and a release message and keep a retain count which can be queried by sending a retainCount message. A newly allocated object created with alloc or copy has a retain count of one. Sending that object a retain message increments the retain count, while sending it a release message decrements the retain count. When an object's retain count reaches zero, it is deallocated similar to a C++ destructor. dealloc is not guaranteed to be invoked.
Starting with Objective-C 2.0, the Objective-C runtime implements an optional garbage collector. In this model, the runtime turns Cocoa reference counting operations such as "retain" and "release" into no-ops. The garbage collector does not exist on the iOS implementation of Objective-C 2.0. Garbage Collection in Objective-C runs on a low-priority background thread, and can halt on Cocoa's user events, with the intention of keeping the user experience responsive.
In 2011, the LLVM compiler introduced ARC (Automatic Reference Counting), which replaces the conventional garbage collector by performing static analysis of Objective-C source code and inserting retain and release messages as necessary.
Read more about this topic: Cocoa (API)
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