Declare passed parameter to var in Swift

Question

I want to create a function that takes 2 parameters and changes their values. Example:

var geometricSprite = SKSpriteNode() var geoTouched = true removeTouch(sprite: geometricSprite, bool: getTouched) func removeTouch(sprite: SKSpriteNode, bool: Bool) { sprite.size = 0 bool = false //HERE I GET THE ERROR "Cannot assign to value: 'bool' is a 'let' constant" }

So again bool = false gives an error because 'bool' is a let constant. I never declared that bool as a let so I do not know how to fix this error.

Are you just trying to assign to it or are you also looking to change the value of getTouched that was passed to it? — David Rönnqvist
– David Rönnqvist, Commented Nov 7, 2017 at 17:31
I am trying to change the value to "false". But the error appears! — Jush KillaB
– Jush KillaB, Commented Nov 7, 2017 at 17:58
You don't need to have declared it. In the Swift manual, "Functions": "Function parameters are constants by default." — Ssswift
– Ssswift, Commented Nov 7, 2017 at 19:08

David Rönnqvist · Accepted Answer · 2017-11-07 19:09:35Z

Since your are looking to both pass and argument to the function and modify the value that was passed in you need to define that parameter as inout:

func removeTouch(sprite: SKSpriteNode, bool: inout Bool)

That also changes the call site to require a & before the value being passed in

removeTouch(sprite: geometricSprite, bool: &getTouched)

That said, depending on what you are doing with the value that was passed to the function, you are probably better off returning the new value.

func removeTouch(sprite: SKSpriteNode, bool: Bool) -> Bool

dfrib · Accepted Answer · 2017-11-07 19:14:02Z

Unless you specify a function parameter as inout, you may not mutate it.

SKSpriteNode() is of type class, which is a reference type: not marking it inout means you may not mutate the reference itself, whereas you may, however, mutate mutable (accessible) members of the reference type (accessible via the non-mutable reference).

bool, on the other hand, is a value type, which means that you may not mutate it when supplying it as an argument, unless you explicitly mark it as inout.

A hopefully instructive example:

class MyReferenceType { var valueTypeMember: Int = 0 } struct MyValueType { var valueTypeMember: Int = 0 } func mutateMyReferenceAndValueInstances( _ ref: MyReferenceType, _ val: inout MyValueType) { ref.valueTypeMember = 42 val.valueTypeMember = 43 } let ref = MyReferenceType() var val = MyValueType() print(ref.valueTypeMember, val.valueTypeMember) // 0 0 mutateMyReferenceAndValueInstances(ref, &val) print(ref.valueTypeMember, val.valueTypeMember) // 42 43

See e.g. the Language Guide - Functions for details regarding in-out parameters.

Collectives™ on Stack Overflow

Declare passed parameter to var in Swift

2 Answers 2

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Linked

Related