Memory Management: “reference counting” methods

You manipulate the retain count (take and relinquish ownership) using a variety of methods:

alloc

Allocates memory for an object, and returns it with retain count of 1.

You own objects you create using any method that starts with the word alloc or with the word new.

copy

Makes a copy of an object, and returns it with retain count of 1.

If you copy an object, you own the copy. This applies to any method that contains the word copy where “copy” refers to the object being returned.

retain

Increases the retain count of an object by 1.

Takes ownership of an object.

release

Decreases the retain count of an object by 1.

Relinquishes ownership of an object.

autorelease

Decreases the reference count of an object by 1 at some stage in the future.

Relinquishes ownership of an object at some stage in the future.

The rules for memory management are summarized as follows (see also “Memory Management Rules”):

• Within a given block of code, the number of times you use copy, alloc and retain should equal the number of times you use release and autorelease.

• You only own objects you created using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message.

• Implement a dealloc method to release the instance variables you own.

• You should never invoke dealloc directly (other than when you invoke super’s implementation in a custom dealloc method).

http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/MemoryMgmt/Articles/mmPractical.html