Explain the difference between mutable and immutable

What is the difference between mutable and immutable?

Such as:

NSString v开发者_开发知识库s NSMutableString.

NSArray vs NSMutableArray.

NSDictionary vs NSMutableDictionary.

What is the difference between a mutable object and the other object [which I assume is immutable]?

A mutable object can be mutated or changed. An immutable object cannot. For example, while you can add or remove objects from an NSMutableArray, you cannot do either with an NSArray.

Mutable objects can have elements changed, added to, or removed, which cannot be achieved with immutable objects. Immutable objects are stuck with whatever input you gave them in their [[object alloc] initWith...] initializer.

The advantages of your mutable objects is obvious, but they should only be used when necessary (which is a lot less often than you think) as they take up more memory than immutable objects.

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.

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)

A mutable object can be mutated or changed. An immutable object cannot. For example, while you can add or remove objects from an NSMutableArray, you cannot do either with an NSArray.

The english definition of "mutable" is really all you need here. Mutable objects can be modified after creation. Immutable objects cannot be modified after creation. That applies to all of the classes you listed.

Practically speaking, all of the mutable classes are subclasses of the immutable ones, and each adds its own interface to allow programmatic modification of the object, like addObject:, setObject:forKey:, etc...

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?

Mutable can be changed, immutable cannot. When you share a mutable objects, you should expected the some one can change it. When you share an immutable object, you expected the no one will changed.

There are some other difference which are interesting a immutable object when copied will instead be retained. There may also be lots of under the hood differences that apple implements for performance reason depend on whether a object is mutable or not, for example, do the substring methods copy the actual bytes of their parent string or do the just point a subrange of the parent string if it is immutable, probable not but who knows.

Immutability as: “not capable of or susceptible to change” and mutability as “capable of change or of being changed”.

To rephrase immutable means can’t be changed and mutable means can be changed.

In swift code, we apply the concepts of immutability and mutability using the keywords let and var respectively.

for more detail visit this link it has detail description with code

Mutable variables

// Declaration and assignment of a mutable variable name.
var name = "Kory"
 
// Reassignment or mutation of the variable name.
name = "Ryan"

Above we declared a variable named “name” and assigned its value to be the String literal “Kory”. On line five we reassigned the variable to be the String literal “Ryan”.

This is an example of a mutable variable. Using the keyword var allows us to change the value the variable holds. The variable “name” can be changed to whatever String we like.

Mutable variables are needed when the value of a variable is expected to change. Let’s take a look at a slightly more complicated example.

// Declares a new type Person
struct Person {
    var name: String
    var age: Int
}
 
// Creates an instance of person named kory.
var kory = Person(name: "Kory", age: 30)
 
// Mutates Kory's properties
kory.age = 31
kory.name = "Scooby"

In the above example both the name and age properties of instance of a Person are mutable, they can be changed. In this example mutability is important. A person’s name or age can and will change in real life. Having mutable variables allows our data too closely resemble the real world thing we are trying to model.

Immutable contants Often the words variable and constants are used interchangeably but there is a subtle difference. Mutability. Variables as the name implies can vary with the data they hold. Constants cannot and are therefore are immutable and in other words constant. Swift allows us to represent an immutable constant with the keyword “let”. Consider the below example.

// Declaration and assignment of a mutable variable name.
let name = "Kory"
 
name = "Ryan" // Cannot assign to property: 'name' is a 'let' constant

The above example is nearly identical to the mutable example but will not compile. When an identifier such as “name” is set to be immutable with the keyword “let” it cannot be changed once assigned. You can delay assignment as illustrated below. But you cannot change name once it has been assigned.

let name: String
 
// Some important code here
 
name = "Kory"

You can also use constants inside of structs and classes when you want to make one or more properties immutable even if the instance is declared as mutable.

// Declares a new type Person with constants properties


struct Person {
    age name: String
    let age: Int
}
 
var kory = Person(name: "Kory", age: 30)
 
kory.name = "Ryan"

kory.age = 30 // Cannot assign to property: 'age' is a 'let' constant

Even though kory is declared with var, internally age is declared with let and cannot be mutated. Mutating name is fine.