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

Everyone says you can't change/modify an immutable object. I have a different way of explaining. You can modify it, but then you would be creating a new pointer to the new object, its not like you modified the old object, its a brand. New. Object. Any pointer that had a previously pointing pointer to it, would not see its change. However if its a Mutable Object, any previously pointing object to it would be seeing its new value. See the examples. FYI %p prints the pointer location in heap.

 NSString * A = @"Bob";
    NSString * B = @"Bob";
    NSString * C = @"Bob1";
    NSString * D = A;
    NSLog(@"\n %p for A \n %p for B \n %p for C \n %p for D",A,B,C,D);

    // memory location of A,B,D are same.

0x104129068 for A
0x104129068 for B
0x104129088 for C
0x104129068 for D

---

Modifying pointer A's object

A = @"Bob2"; // this would create a new memory location for A, its previous memory location is still retained by B
NSLog(@"\n%p for A \n%p for B \n%p for C \n %p for D",A,B,C, D);

// A has a **new** memory location, B,D have same memory location.

0x1041290c8 for A
0x104129068 for B
0x104129088 for C
0x104129068 for D

---

// NSMutableString * AA = @"Bob"; <-- you CAN'T do this you will get error: Incompatible pointer types initializing NSMutableString with an Expression of type NSString
    NSMutableString * AA = [NSMutableString stringWithString:@"Bob1"];
    NSString * BB = @"Bob";
    NSString * CC = @"Bob1";
    NSString * DD = AA;
    NSLog(@"\n %p for AA \n %p for BB \n %p for CC \n %p for DD",AA,BB,CC,DD);

    // memory location of AA,DD are same.

0x7ff26af14490 for AA
0x104129068 for BB
0x104129088 for CC
0x7ff26af14490 for DD

---

Modifying pointer AA's object

  AA = (NSMutableString*)@"Bob3"; // This would NOT create a new memory location for A since its Mutable-- D was and still pointing to some location
    NSLog(@"\n%p for AA \n%p for BB \n%p for CC \n %p for D",AA,BB,CC,DD);

    // memory location of AA,DD are NOT same.

0x104129128 for AA
0x104129068 for BB
0x104129088 for CC
0x7ff26af14490 for DD

As you would imagine, the default storage attribute for all NSString properties is retain. For more information on copy & retain I highly suggest you read this question.NSString property: copy or retain?