what is difference between mutable and immutable

Question

what is difference mutable and immutable

like

NSString and NSMutableString.

NSArray and NSMutableArray.

NSDictionary and NSMutableDictionary

Answer 1

Mutable objects can be modified, immutable objects can't.

Eg: NSMutableArray has addObject: removeObject: methods (and more), but NSArray doesn't.

Modifying strings:

NSString *myString = @"hello";
myString = [myString stringByAppendingString:@" world"];

vs

NSMutableString *myString = @"hello";
[myString appendString:@" world"];

Mutable objects are particularly useful when dealing with arrays,

Eg if you have an NSArray of NSMutableStrings you can do:

[myArray makeObjectsPerformSelector:@selector(appendString:) withObject:@"!!!"];

which will add 3 ! to the end of each string in the array.

But if you have an NSArray of NSStrings (therefore immutable), you can't do this (at least it's a lot harder, and more code, than using NSMutableString)

Answer 2

The basic difference is:

• NSStrings cannot be edited, only reassigned. This means when the value of an NSString changes, it is actually pointing to a new location in memory.

• NSMutableString objects can be edited and maintain the same pointer.

A common practical difference is:

• If you create 1 NSString and then assign another one to it, then edit either one of them, they will now be pointing to different strings.

• If you do the same thing with NSMutableStrings, but then just edit one of them (not reassign it), they will both be pointing to the newly edited object.