Swift pass struct by reference?

Question

I've looked to similar questions but I haven't seen an answer that I am satisfied with.

Is it possible or advisable to pass structs by reference? If so how?

Here is a code as a reference for examples:

struct MyData {     var contentId: Int = 0     var authorId: Int = 0     var image: UIImage = UIImage(named: "myimage") } 

As you see my main reason of doing this is because not having my image multiplying all over the place.

Answer 1

Structs can be passed by reference using the inout keyword and the & operator.

struct Test {     var val1:Int     let val2:String      init(v1: Int, v2: String) {         val1 = v1         val2 = v2     } }  var myTest = Test(v1: 42, v2: "fred")  func change(test: inout Test) {     // you can mutate "var" members of the struct     test.val1 = 24      // or replace the struct entirely     test = Test(v1: 10, v2: "joe") } change(test: &myTest) myTest // shows val1=10, val2=joe in the playground 

This practice is discouraged unless you can prove it's the only way to get the performance you need in a critical situation.

Note that you won't save the burden of copying the UIImage by doing this. When you put a reference type as a member of a struct, you still only copy the reference when you pass it by value. You are not copying the contents of the image.

Another important thing to know about struct performance is copy-on-write. Many built in types like Array are value types, and yet they're very performant. When you pass around a struct in Swift, you don't undergo the burden of copying it until you mutate it.

Check out the WWDC video on value types to learn more.