Strange syntax when try to iterate through the array of structs in swift

Question

I have a simple array of structs

var fields: [ProfileField]?

when I try to iterate through this array, swift shows me error on this code:

guard let _fields = fields else {return}
for field in _fields {
}

the error is:

Cannot use mutating getter on immutable value: 'field' is a 'let' constant

And this code compiles well:

for var field in _fields {
}

Why do I need to declare field as var?

I think you need to reveal what you're up to in your for loop — Mike Pollard
– Mike Pollard, Commented Oct 30, 2015 at 14:29
Yes, when your struct was a class it was a reference. Now it's a struct and a value, which if you declare with let you can't change. — Mike Pollard
– Mike Pollard, Commented Oct 30, 2015 at 14:31
Possible duplicate of stackoverflow.com/questions/29777891/…. — Martin R
– Martin R, Commented Oct 30, 2015 at 14:38

Paul Cantrell · Accepted Answer · 2015-10-30 15:37:06Z

2

A “mutating getter” in Swift is a property whose get block has the mutating modifier. For example, if your ProfileField looked like this:

struct ProfileField {
    var accessCount: Int = 0

    var x: Int {
        mutating get {    // ← mutating getter here
            accessCount++
            return x
        }
    }
}

…then this code would generate your “Cannot use mutating getter on immutable value” error:

for field in _fields {
    print(field.x)
}

Even though it doesn’t look like field.x modifies field, it does: it increments accessCount. That’s why you must say var field to make field mutable. (For loop iterators are let by default.)

Without seeing either your ProfileField or the body of your for loop, it’s impossible to say exactly why this occurs in your case. If you are not using mutating get in ProfileField itself, it may be happening in a struct nested within ProfileField.

edited Oct 30, 2015 at 15:37

answered Oct 30, 2015 at 15:31

Paul Cantrell

9,3592 gold badges44 silver badges50 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

ShurupuS Over a year ago

Great! Thank you so much for the "Even though" - now I understand that accessCount is changing! Best of the best!

Paul Cantrell Over a year ago

Glad it helped. Further surprising detail (that makes sense when you think about it): it’s not accessCount++ that causes the compile error, but the word mutating itself. If a function modifies the struct, you have to declare it mutating, but the converse is not true. You can declare something mutating just to reserve the right to modify the struct even though you don’t. It’s an API choice.

Collectives™ on Stack Overflow

Strange syntax when try to iterate through the array of structs in swift

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related