What is the logic behind having a mutable and immutable version of classes like NSArray, NSDictionary etc in Objective C?

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Why do common collection classes in Objective C like NSString, NSArray, NSDictionary etc have a mutable as well as an immutable version. What is the logic behind defining them separately? Performance, memory management or anything else?


The immutable versions of the classes exist because an immutable object is, in and of itself, a unique identifier for a particular state. I.e. if you have an NSArray of 100 NSString instances, that NSArray instance can be treated as idempotent for any one of those strings.

As well, the immutability means that a change cannot happen after the state has been vended. For example, NSView's subviews method returns an immutable array, thus ensuring that the caller isn't going to play games with the contents (nor even expect to be able to). Internally, NSView could choose to return the [likely] NSMutableArray that contains the subviews (since it is internally mutable) and the typecast to NSArray means the caller can't manipulate the contents without an evil cast or bad compiler warning. (This may or may not actually be the real implementation, btw -- but this pattern is used elsewhere).

Immutability also means that enumeration and/or traversal can be done without risk of a state change in the middle. Similarly, many immutable classes are also explicitly thread safe; any number of threads can simultaneously read the immutable state, often without need for a lock.