Swift array reference issue

Question

Here is my code:

    var productsList:[Product]

    switch category {
    case 0:
        productsList = self.sharedInstance.productsList1
    case 1:
        productsList = self.sharedInstance.productsList2
    default:
        productsList = self.sharedInstance.productsList3
    }

    for productDic in products {
        let product = Product()
        // Set product attributes here.
        productsList.append(product)
    }

    println(productsList)
    println(self.sharedInstance.productsList1)
    println(self.sharedInstance.productsList2)
    println(self.sharedInstance.productsList3)

The output:

productsList contains some added content
productsList1, productsList2 and productsList3 are all empty.

In my understanding, productsList should refer to one of productsList1, productsList2 and productsList3 depending on the category.

Can someone explain why productsList1, productsList2 and productsList3 are all empty?

Thanks in advance.

Array is value type so it copy on assignment. The behaviour you expected is for reference type. — Bryan Chen
– Bryan Chen, Commented Aug 17, 2015 at 2:10

Tom Elliott · Accepted Answer · 2015-08-17 02:21:51Z

3

Assigning the array from sharedInstance results in the array being copied.

You would need to re-assign the updated array to the sharedInstance array after making the update.

As a minimal example:

var list:[String] = []

list.append("String1")
list.append("String2")
list.append("String3")

var list2:[String] = list

list2.append("String4")

println(list)
println(list2)

Will result in the output:

"[String1, String2, String3]" "[String1, String2, String3, String4]"

You would need to add a re-assignment to get the lists matching:

//...
list2.append("String4")
list = list2
println(list)
println(list2)

You can also demonstrate the copy by printing the address of each list after assignment and showing they are different.

print(unsafeAddressOf(list))
print(unsafeAddressOf(list2))

Or you can check in the debugger.

If you want to operate specifically on the sharedInstance copies of your arrays you could do one of the following:

Instead of 3 separate properties, you could put them in a parent array property and select an index in your switch statement, rather than a new variable
Wrap your arrays in a custom class, instances of which are passed by reference.

You could also use NSMutableArray, although it's probably not the best of ideas to mix the old and new like that.

edited Aug 17, 2015 at 2:21

answered Aug 17, 2015 at 2:02

Tom Elliott

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2 Comments

yibuyiqu Over a year ago

Thanks for answering. Is there a way to explicitly force the variable to take the same reference? I think it is annoying to assign it back, and make the code not concise.

yibuyiqu Over a year ago

Thanks Tom for the recommendations! I think I will use the parent array option.

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