How to pass structure by reference?

Question

If I have some existing struct, but I want to use "Reference" behavior, how do I achieve that?

I can write some simple class-holder like

class Box<T> {
    var value: T
    init(_ value: T) {
        self.value = value
    }
}

I guess there must be ready class in the standard library, but I didn't find it.

I want to store that reference in my class, so inout parameter isn't what I need.

Would a `class` be more suitable for your purposes? They're passed by reference. Instances of `struct` are designed to be passed by value. — sapi, Oct 18 '14 at 06:27
That would solve my issue, but that means that I have to duplicate existing framework structure like CGRect and implement conversions back and forth. I want to avoid duplication if possible. — vbezhenar, Oct 18 '14 at 06:34
@sapi: Everything in Swift is always passed by value, unless the parameter is marked `inout` in which case it is always passed by reference. — newacct, Oct 19 '14 at 03:58
@newacct Basically, `inout` parameter is not "call-by-reference", but rather ["call-by-copy-restore"](http://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_copy-restore). — rintaro, Oct 20 '14 at 04:08

score 7 · Accepted Answer · answered Nov 06 '14 at 13:57

7

For me the best variant was using class-holder:

class Ref<T> {
  var value: T

  init(_ value: T) {
    self.value = value
  }
}

answered Nov 06 '14 at 13:57

vbezhenar

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score 6 · Answer 2 · edited Jul 28 '16 at 02:26

You can, but you should not store it in your class.

struct Size {
    var width:Float
    var height:Float
}

class Rect {
    var sizeRef:UnsafeMutablePointer<Size>
    init(_ size:UnsafeMutablePointer<Size>) {
        self.sizeRef = size
    }
    func doubleSize() {
        self.sizeRef.memory.height *= 2.0
        self.sizeRef.memory.width *= 2.0
    }
}  

var size = Size(width: 20.0, height: 20.0)
let rect = Rect(&size)
rect.doubleSize()
println("size: \(size.width) x \(size.height)") // -> size: 40.0 x 40.0

Because, usually, struct is allocated from "stack" memory, when you do like this:

func makeRect() -> Rect {
    var size = Size(width: 20.0, height: 20.0)
    return Rect(&size)
}

let rect = makeRect()

rect.sizeRef no longer points valid memory. UnsafeMutablePointer is unsafe in a literal sense.

Arutyun Enfendzhyan · Answer 3 · 2021-09-17T19:54:18.383

I use this:

@propertyWrapper public struct Inout<T> {
    public init(wrappedValue: T) {
        storage = .init(value: wrappedValue)
    }
    
    private let storage: Storage
    
    public var wrappedValue: T {
        nonmutating get { storage.value }
        nonmutating set { storage.value = newValue }
    }
    
    private class Storage {
        init(value: T) {
            self.value = value
        }
        
        var value: T
    }
}

Example:

struct Example { 
    init(value: Inout<Int>) { 
        _value = value
    }
    
    @Inout var value: Int
    
    func performAction() { 
        value += 19
    }
}
let value = Inout(wrappedValue: 50)
let example = Example(value: value)
example.performAction()
print(value.wrappedValue) // 69

How to pass structure by reference?

3 Answers3

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