Why Can't I Update an Object in Array using a "helper" Variable?

Question

I'm asking this out of curiosity to understand Swift.

I'm trying to update objects in an array situated in another class.

I have two cases (the other one works, and other one doesn't)

Working solution:

    Data.tripModels[0].title = "lol"

Not working:

    var trip = Data.tripModels[0]
    trip.title = "lol"

To help you understand:

    Data = the other class
    tripModels = the array in Data class, holding the objects
    title = a property of tripModel in tripModels array

Why is the 2. not working? :(

Are the items in the tripModels array instances of a struct? — vacawama
– vacawama, Commented May 21, 2019 at 16:44
Then @vadian's answer explains the issue. structs are value types, so you're updating a copy. — vacawama
– vacawama, Commented May 21, 2019 at 17:02

vadian · Accepted Answer · 2019-05-21 16:53:12Z

1

The 2. does not work because due to value semantics (the type of tripmodel is a struct) the line

var trip = Data.tripModels[0]

assigns a copy of the item in the array to trip and

trip.title = "lol"

updates the copy but not the item in the array.

Please read Structures and Enumerations are Value Types in the Swift Language Guide

edited May 21, 2019 at 16:53

answered May 21, 2019 at 16:47

vadian

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2 Comments

Pete Silva Over a year ago

Thanks a lot for your fast response. Now that you said it it makes sense to me also... Would it work if I would leave the [0] out of the variable? Or would it just save a copy of the whole array instead of a reference?

vadian Over a year ago

It creates also a copy of the array. But you can reassign the changed item: var trip = Data.tripModels[0]; trip.title = "lol"; Data.tripModels[0] = trip

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