Cannot use mutating member on immutable value of type

Question

I have following struct:

public protocol SuperModel {
    // empty protocol
}
struct ModelOne: SuperModel {
    struct SubModelOne {
        var someVar: Double
        var othervar: Double?
    }
    var sub: SubModelOne?
    mutating func setSub(sub: SubModelOne) {          
        self.sub = sub
    }
}

In my class, I want to use this struct like that:

final class SomeClass: SuperClass {
    var data: SuperModel
    init() {
        self.data = ModelOne()
    }
    func someFunc() {
        (self.data as! ModelOne).setSub(ModelOne.SubModelOne(someVar: 2, otherVar: 1))
    }
}

I get following error: Cannot use mutating member on immutable value of type 'ModelOne'. Why is that so and how can I fix this?

What is NetworkCheckModel? A self-contained example would be useful. — Martin R, Aug 04 '16 at 09:52
try this var data = self.data as! ModelOne data.setSub(ModelOne.SubModelOne(someVar: 2, othervar: 1)) — Sahil, Aug 04 '16 at 09:57
@MartinR sorry I was in a rush and forget to change it. Edit was made — j0h4nn3s, Aug 04 '16 at 10:50
@SahilBeri this works, but that means i would have a copy of it. Can I do `self.data = data` do copy it back? — j0h4nn3s, Aug 04 '16 at 10:53
You can do it self.data = data but then you will not able to call setSub func using only self.data . only then you can call only those which is define in SuperModel protocol. — Sahil, Aug 04 '16 at 11:24
> by doing this var data = self.data as! ModelOne you are getting object of ModelOne which reference is you assigning to data variable and calling the setSub function. - @SahilBeri you are very close to right answer, but by some mistake. You think that structs are reference types, but if so - original code should work. Structs are value types. — Yury, Aug 04 '16 at 11:32
basically a copy of the ModelOne is made, and this new copy is assigned to data. — Sahil, Aug 04 '16 at 11:36

score 51 · Accepted Answer · answered Aug 04 '16 at 11:25

When you apply type casting to value types (such structs), if succeed, you receive immutable copy of requested value:

(self.data as! ModelOne) // this is copy of data

The only way (as known to me) how you can mutate values that need to be casted - reassign value (as @Sahil Beri pointed you need declare variable):

func someFunc() {
    if var data = data as? ModelOne {
        data.setSub(ModelOne.SubModelOne(someVar: 2, otherVar: 1))
        self.data = data // you can do this since ModelOne conforms to SuperModel
    }
}

Neat solution. Also works for when using protocols to customize UI. — inokey, May 31 '18 at 09:01

score 3 · Answer 2 · edited Nov 17 '22 at 04:39

Use like this,

struct UserAttributes {
    var name:String?
    var organizationID:String?
    var email:String?
    
    mutating func parseUserAttributes(attribues:[AWSCognitoIdentityProviderAttributeType])->UserAttributes{
        
        for type in attribues{
            if type.name == "name"{
                name = type.value
            }else if(type.name == "family_name"){
                organizationID = type.value
            }else if(type.name == "custom:role_id"){
                role = type.value
            }else if(type.name == "email"){
                email = type.value
            }
        }
    }
}

In some other file call like this,

var userAttributes = UserAttributes()
userAttributes = userAttributes.parseUserAttributes(attribues:attributes)

score 1 · Answer 3 · answered Aug 04 '16 at 10:58

Problem is that you have declared data as SuperModel but allocate it as ModelOne. Declare data as ModelOne. Then the problem goes away.

final class SomeClass: SuperClass {
    var data: ModelOne
    init() {
        self.data = ModelOne()
    }
    func someFunc() {
        (self.data).setSub(ModelOne.SubModelOne(someVar: 2, otherVar: 1))
    }
}

score 0 · Answer 4 · answered Aug 04 '16 at 11:11

0

First downcast the self.data to ModelOne then call setSub function

 if var data = self.data as? ModelOne {
   data.setSub(ModelOne.SubModelOne(someVar: 2, othervar: 1))
 }

answered Aug 04 '16 at 11:11

Sahil

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as @ShadowOf wrote in his answer, I need to copy the value back to self.data – j0h4nn3s Aug 04 '16 at 12:14
you can do this by self.data = data as i mentioned in comments. :) – Sahil Aug 06 '16 at 09:07

score 0 · Answer 5 · edited May 23 '17 at 12:09

0

@Shadow of is right. You try to mutate a temporary structure which is impossible and most of the time useless as it will be released once the mutation done. It's in fact a similar issue to trying to modify the return struct of a function. (see answer here : Cannot assign to property: function call returns immutable value)

edited May 23 '17 at 12:09

Community

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answered Aug 04 '16 at 12:39

Bioche

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score 0 · Answer 6 · edited Jan 18 '17 at 17:03

0

In Swift 3, in my case, I was able to resolve the error just by changing struct to a class object.

edited Jan 18 '17 at 17:03

shim

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answered Oct 18 '16 at 20:22

Naishta

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27

Should not do that in Swift. – lee Oct 19 '16 at 07:28
3

Yes, of course you fix it! But this isn't a smart approach to do it. Check https://developer.apple.com/videos/play/wwdc2015/408/ – Daniel Molina Oct 03 '17 at 07:24
This solved my issue where I was looping through and array of objects that contained another object that i needed to modify. – Wez Jan 31 '19 at 11:27

Cannot use mutating member on immutable value of type

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