Editing Cocoa (API) (section)

==Memory management==
One feature of the Cocoa environment is its facility for managing dynamically allocated memory. [[Foundation Kit]]'s NSObject class, from which most classes, both vendor and user, are derived, implements a [[reference counting]] scheme for memory management. Objects that derive from the NSObject root class respond to a <code>retain</code> and a <code>release</code> message, and keep a retain count. A method titled <code>retainCount</code> exists, but contrary to its name, will usually not return the exact retain count of an object. It is mainly used for system-level purposes. Invoking it manually is not recommended by Apple.

A newly allocated object created with <code>alloc</code> or <code>copy</code> has a retain count of one. Sending that object a <code>retain</code> message increments the retain count, while sending it a <code>release</code> message decrements the retain count. When an object's retain count reaches zero, it is deallocated by a procedure similar to a C++ ''destructor''. <code>dealloc</code> is not guaranteed to be invoked.

Starting with Objective-C 2.0, the Objective-C runtime implemented an optional [[Objective-C#Garbage collection|garbage collector]], which is now obsolete and deprecated in favor of [[Automatic Reference Counting]] (ARC). In this model, the runtime turned Cocoa [[reference counting]] operations such as "retain" and "release" into [[no-op]]s.  The garbage collector does not exist on the [[iOS]] implementation of Objective-C 2.0. Garbage collection in Objective-C ran on a low-priority background thread, and can halt on Cocoa's user events, with the intention of keeping the user experience responsive.  The legacy garbage collector is still available on Mac OS X version 10.13, but no Apple-provided applications use it.

In 2011, the [[LLVM]] compiler introduced [[Automatic Reference Counting]] (ARC), which replaces the conventional garbage collector by performing static analysis of Objective-C source code and inserting retain and release messages as necessary.