retainrelease

Retainrelease, also known as the retain/release memory management pattern, is a memory management mechanism used in manual reference counting systems, most notably in Objective-C and the Cocoa frameworks before Automatic Reference Counting (ARC) was introduced. In this model, objects maintain a numeric reference count that represents ownership by parts of the program. Sending a retain message to an object increases its count, while sending a release message decreases it. When the count reaches zero, the object's dealloc method is invoked and its memory is reclaimed. To allow temporary ownership without immediate deallocation, objects can be sent an autorelease message, which places them in the current autorelease pool and releases them automatically when the pool is drained, typically at the end of the run loop iteration.

Ownership rules are central to the retainrelease pattern. You own objects you create with alloc/init, new, copy, or mutableCopy, and you are responsible for releasing or autoreleasing them when you are finished. If you did not create or retain an object, you should not release it. If you want to transfer ownership to another part of the program, you may retain the object to keep it alive and later release it when it is no longer needed.

Common pitfalls include memory leaks from forgotten releases and retain cycles where two or more objects hold strong references to each other, preventing deallocation. Tools such as static analyzers and careful design (for example, using weak references) help mitigate these issues, especially in codebases that mix manual retainrelease with later ARC.

In practice, retainrelease has largely been superseded by ARC, which automatically inserts retain and release calls. Manual retainrelease code remains in older codebases and requires disciplined ownership management.