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 { var value: String init(value: String) { self.value = value } } class B { var v1: A? var v2: A = A(value: "2") private var v: A { return v1 ?? v2 } var value: String { get { return v.value } set { v.value = newValue } } }

This code is simple and it works. Since both the A and B have a member "value", I make it a protocol like this:

protocol ValueProvider { var value: String {get set} } class A: ValueProvider { var value: String init(value: String) { self.value = value } } class B: ValueProvider { var v1: ValueProvider? var v2: ValueProvider = A(value: "2") private var v: ValueProvider { return v1 ?? v2 } var value: String { get { return v.value } set { v.value = newValue // Error: Cannot assign to the result of the expression } } }

If I change the following code

v.value = newValue

to

var v = self.v v.value = newValue

It works again!

Is this a bug of Swift, or something special for the property of protocols?

Martin R · Accepted Answer · 2015-04-19 11:39:48Z

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).

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