Swift Struct instance get changed on updating it's value

Question

I have observed that when we do change a property value of struct, then a new struct object is created.

struct StructureTest {
    var i: Int

    mutating func changeValue(_ val: Int) {
        i = val
    }
}

var st = StructureTest(i: 10) {
    didSet{
        print("struct changed")
    }
}

print("before: \(st)")
st.changeValue(20)
print("after: \(st)")

Output:

before: StructureTest(i: 10)   
struct changed
after: StructureTest(i: 20)

Main thing I have noticed that after changing it's property value. A new struct object is created and assigned to var st.

I do know what are "value types" and what is "copy on assign" feature.

But I am unable to understand that why is it happening here? Maybe I am missing something here? Let me know if you do know the reason for this new struct instance creation.

Along side with this I have observer one more thing that:

If I have a array of structs. Like as:

struct StructureTest {
    var i: Int

    mutating func changeValue(_ val: Int) {
        i = val
    }
}

var arrStructs = [StructureTest(i: 10), StructureTest(i: 20)] {
    didSet {
        print("arrStructs changed")
    }
}

arrStructs[0].changeValue(30)

Output:

arrStructs changed

I am unable to understand that why is array modifying?

As much I can understand from "copy of write" feature on value types. It should happen when an array is modified and array capacity requires to create a new array but in this case array modification reason is not clear to me.

Do let me know If you do know the reason behind that or if you can provide me any reference for clarification.

Sorry for my grammatical mistakes. Hope the essence of problem is clear to you.

Answer 1

The structs have value semantics, so when you mutate a struct you have assigned a new value to it; ie you changed the value that the variable holds. Array in swift is a generic struct and it therefore also has value semantics (unlike most languages where array is a reference). Therefore if you mutate any element of the array, you have changed the value of the entire array.

Answer 2

I have read the comments/replies to the post. One more thing/finding, I would like to add here is that when you modify a struct (e.g. changing its property) the entire struct is reinitialized. However the address of that struct (value) remains the same. The new struct is stored at the same location in the stack.

And when we copy a struct, e.g.

var s = SomeStruct()
var copyOfS = s

The address of the "copyOfS" is changed/different (as expected obviously).