The final word on NSStrings: Mutable and Immutable

Question

I\'ve read in several books... and online... about immutable and mutable strings. They claim \"immutable strings\" can\'t be changed. (But they never define \"change\".)

Answer 1

An immutable string (or indeed, any immutable object) is one that can't be changed at all after initialisation.

Here's the advantage of using immutable strings, in (pseudo-)pseudo-code:

// if using mutable strings
let a = "Hello World";
let b = a;
a.replace("Hello", "Goodbye");

Now what does a hold? "Hello World"? "Goodbye World"? Now what does b hold?

If strings are mutable, then it might hold "Goodbye World". This may be what you want. It might not be.

With immutability, it becomes very clear that b still points to the original string, because the replace method can't have changed the actual string, but rather returned a reference to a new string.

You might not see much benefit in this example. But if a and b are referenced and used in different parts of the code, or in different threads, having immutable references removes a lot of potential errors.