Why are properties of an immutable object mutable in Swift?

Question

In Swift, we denote an immutable variable with let.

What I don't understand is why you change their properties. For example:

let lbl = UILabel()
lbl.textAlignment = .Right()

Why can you change textAlignment? By virtue of mutating the property, haven't we also mutated the variable lbl that was supposed to be constant?

The variable/binding cannot be re-assigned to a different value; this alone says nothing about the mutability of the object that results from evaluation of the variable. This is the same as final/readonly variables/fields in Java and C#. — user2864740
– user2864740, Commented Sep 13, 2014 at 21:00

erdekhayser · Accepted Answer · 2014-09-13 21:00:33Z

16

According to the Swift Programming Language, the properties of constant structs are also constant, but constant classes can have mutable properties.

In their words,

If you create an instance of a structure and assign that instance to a constant, you cannot modify the instance’s properties, even if they were declared as variable properties...

The same is not true for classes, which are reference types. If you assign an instance of a reference type to a constant, you can still change that instance’s variable properties.

answered Sep 13, 2014 at 21:00

erdekhayser

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1 Comment

Jason Renaldo Over a year ago

Thanks, explains it perfectly. Will accept when I can.

newacct · Accepted Answer · 2014-09-14 05:49:42Z

3

Class types are reference types -- the value is a pointer to an object. Not being able to change it simply means not changing the reference, to point to another object. It does not have anything to do with what you can do with the object being pointed to.

answered Sep 14, 2014 at 5:49

newacct

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