Swift: Failed to assign value to a property of protocol?

Question

Class A provides a string value. Class B has two members of A type inside itself, and provide a computed property \"v\" to choose one of them.

class A {
             


        

Answer 1

You have to define the protocol as a class protocol:

protocol ValueProvider : class {
    var value: String {get set}
}

Then

var value: String {
    get { return v.value }
    set { v.value = newValue }
}

compiles and works as expected (i.e. assigns the new value to the object referenced by v1 if v1 != nil, and to the object referenced by v2 otherwise).

v is a read-only computed property of the type ValueProvider. By defining the protocol as a class protocol the compiler knows that v is a reference type, and therefore its v.value property can be modified even if the reference itself is a constant.

Your initial code example works because there the v property has the type A which is a reference type.

And your workaround

set {
    var tmp = v1 ?? v2
    tmp.value = newValue
}

works because (read-write) properties of variables can be set in any case (value type or reference type).