Cannot use mutating member on immutable value of type - swift

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?

Answer 1

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
}
}

Answer 2

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)

Answer 3

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))
}
}

Answer 4

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))
}

Answer 5

#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)

Answer 6

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