Formal definition of concepts: value type, etc

Using Swift
1

Could anyone provide a formal definition of these concepts please?

- value type
- reference type
- value semantics
- reference semantics
2

No, not really.

3 Likes
3

I'd also add another dimension:

  • realtime safe types

Typically those would be a subset of value types that do not allocate memory or take locks e.g. a struct or a tuple of ints is realtime safe, while a struct or a tuple of strings - is not.

It's tempting to say that identity and === operation strongly hint reference types, but OTOH we can imagine:

struct Value: Identifiable {
    private (set) var value: Int
    var id: ObjectIdentifier { ... }
    static func === (lhs: Self, rhs: Self) -> Bool {
        lhs.value == rhs.value
    }
    func foo() {
//        ....
    }
}

and a modification to ObjectIdentifier.init(...) to accept this value type, then the line is further blurred.

Note, that this is still a value type:

struct S {
    private (set) var ref = NSObject()
    func foo() {
        ...
    }
}

As a very very very rough approximation - instances (themselves) of a reference type would be allocated with malloc. Their fields may in turn be either value or reference types on their own.

2 Likes
4

This post outlines the fundamentals, and section 2 of this paper defines Mutable Value Semantics (a slightly more precise term) and contrasts it with reference semantics. I believe that paper is the current state of the art in understanding these concepts.

8 Likes
5

A contrived counter example:

struct JustAnInt {
    var value: Int
}

extension JustAnInt: RawRepresentable {
    private static var positions: [(Float, Float)] = []
    public var rawValue: (Float, Float) {
        get { Self.positions[value] }
        nonmutating set { Self.positions[value] = newValue }
    }
    public init(rawValue: (Float, Float) = (0, 0)) {
        value = Self.positions.count
        Self.positions.append(rawValue)
    }
}

let ref = JustAnInt()
print("\(ref): \(ref.rawValue)")
let ref2 = ref
ref2.rawValue = (69.0, 105.0)
print("\(ref): \(ref.rawValue)")
6

Why so elaborate :slight_smile:

extension Int {
    func foo() { malloc(1) }
}
42.foo() // "wow, Int is not realtime safe!"
7

There are an infinite number of dimensions of safety that could be used to classify types. Safety for use in realtime systems is certainly an interesting one, but I don't think it's any more related to the value-vs-reference distinction than any other safety property… is it?

8

If extending a type to have some method or computed property with reference semantics makes the type have reference semantics, every type can have reference semantics.

var x = 5

extension Equatable {
  var x: Int { get { x } set { x = newValue } }
}
9

If extending a type to have some method or computed property with reference semantics makes the type have reference semantics…

It doesn't. Every type with value semantics has a notional value, defined by its author. This x property is not part of the notional value of any type.

1 Like