Copying a closure in Swift

Using Swift
1

My understanding is that closures are reference types, so doing something like the following will result in both constants referring to the same instance of the closure:

let closureA = { print("I'm in a closure") }
let closureB = closureA

In Obj-C I believe there was a way to make a copy of a block.

Is there a way to make a copy of a closure in Swift?

2

The copying mechanism in ObjC is just for getting the block off the stack; once itā€™s off the stack, ā€œcopyingā€ just retains the existing value. That detail is something that Swift hides from you.

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3

Closure variables have value semantics. (But are presumably only copyā€onā€write like structures.)

var closure: (Int) -> Void = { print("\($0): Iā€™m still the first closure.") }
closure(1) // 1: Iā€™m still the first closure.

var newVariable = closure
newVariable(2) // 2: Iā€™m still the first closure.

closure = { print("\($0): Iā€™ve been mutated.") }
closure(3) // 3: Iā€™ve been mutated.
newVariable(4) // 4: Iā€™m still the first closure.
4

On second thought, thatā€™s not really mutation but reassignment.

Since closures arenā€™t mutable in the first place, Iā€™m not sure what difference it makes, or why you would need a copy instead of a reference. What is it you are trying to do?

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5

Closures are meant to be blocks of functionality, they don't hold stored state or properties. So I'm also curious what the intention of OP is.

6

That's not true, they do hold state and that state isn't copied when you copy the closure itself. Check out this snippet:

func giveMeAClosureThatHoldsState() -> ( (Int) -> Int ) {
    var state = 0
    return { difference in
        state += difference
        return state
    }
}

let closureWithState = giveMeAClosureThatHoldsState()
let closureCopy = closureWithState
let anotherClosureWithState = giveMeAClosureThatHoldsState()
print(closureWithState(1)) // prints 1
print(closureWithState(1)) // prints 2
print(closureCopy(1)) // prints 3
print(anotherClosureWithState(1)) // prints 1
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7

As others have explained above, closures don't really copy.

Yet there is a closure feature which is somewhat related: the capture list.

The capture list allows a closure to copy the values of captured variables, and this makes a big difference when captured variables are mutable. Compare:

var value = 0
let closure1 = { return value }
let closure2 = { [value] in return value }
print("closure1: \(closure1())") // prints "closure1: 0"
print("closure2: \(closure2())") // prints "closure2: 0"

value = 42
print("closure1: \(closure1())") // prints "closure1: 42"
print("closure2: \(closure2())") // prints "closure2: 0"

See Documentation for more information.

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