How do I make a exact duplicate copy of an array?

arrays swift reference copy
Patrick picture Patrick · Jan 7, 2015 · Viewed 94.9k times · Source

How would I make an exact duplicate of an array?

I am having hard time finding information about duplicating an array in Swift.

I tried using .copy() 

var originalArray = [1, 2, 3, 4]
var duplicateArray = originalArray.copy()

Answer

Nate Cook picture Nate Cook · Jan 7, 2015

Arrays have full value semantics in Swift, so there's no need for anything fancy.

var duplicateArray = originalArray is all you need.

If the contents of your array are a reference type, then yes, this will only copy the pointers to your objects. To perform a deep copy of the contents, you would instead use map and perform a copy of each instance. For Foundation classes that conform to the NSCopying protocol, you can use the copy() method:

let x = [NSMutableArray(), NSMutableArray(), NSMutableArray()]
let y = x
let z = x.map { $0.copy() }

x[0] === y[0]   // true
x[0] === z[0]   // false

Note that there are pitfalls here that Swift's value semantics are working to protect you from—for example, since NSArray represents an immutable array, its copy method just returns a reference to itself, so the test above would yield unexpected results.

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