Return mutable version of struct in Swift

Question

I have a methods that results the struct itself using return self

Unfortunately I am getting the Immutable value of type "Struct name" only has mutable member name 'method name' error.

How can I return such that the struct is mutable?

Answer 1

You can mutate structs just fine, but you have to annotate those mutating methods with mutating.

struct SomeStruct {
    var x:Int = 0
    mutating func increment() {
        x++;
    }
}

Edit:

Let me make some clarifications here. Structs can be mutated:

var structA = SomeStruct()
structA.x // 0
structA.increment()
structA.x // 1

But only if you declared it as var. This will not compile:

let structA = SomeStruct()
// structA.increment() // Compile error

Now when you do something like something.methodReturningStruct().mutatingMethod(), the methodReturningStruct() will actually return a copy of the original struct (by virtue of value types). But since you didn't assign it to a var, it will implicitly be treated as a constant (i.e. let) so you get that compiler error.

You can still do the same thing, but you just have to tell swift to use the struct as a var

var structB = something.methodReturningStruct()
structB.mutatingMethod()