Question

Why should someone ever use the non-NSMutable equivalents of the data structures in Objective-C? When it’s a situation when you need a const object that should not be modified? Does using non-NSMutable classes improve performance in any way? Any other situations?

Answer 1

The two main reasons off the top of my head:

• An object returning a property can be certain nobody will alter it if it’s immutable. The object can therefore return the original instead of making copies all the time. So it’s a memory and performance benefit.
• When writing your own immutable objects, it’s very easy to be thread safe. That naturally flows into being able to write multi-threaded functional-style code which is reasonably efficient and error free.

You also tend to see arguments in favour of the inherent preservation of the original value being useful, especially in terms of semantics and design patterns.

Immutable classes don’t tend to be much more efficient in and of themselves with one exception — if you take an immutable copy of a mutable array, for example, then it’s clear exactly how much storage is needed and exactly that much can be allocated. Because memory allocation costs time, mutable collections tend to keep some spare storage around because they can’t predict how they’re going to grow.