[swift-users] Large Structs, and COW.

[Daniel Tartaglia]

If I have a large struct with lots of sub-structs and I assign to just one field of one of the sub-structs, will the system make a deep copy of the struct or a shallow copy where the unmodified portions of the object still point the same memory as the original struct?

In other words, given this code:

struct SubStruct {
    var a: Int = 0
    var b: Int = 0
}

struct VeryLarge {
    var subStructA = SubStruct()
    var subStructB = SubStruct()
    var subStructC = SubStruct()
    // lots of other stuff
}

func bar(var vl: VeryLarge) -> VeryLarge {
    vl.subStructA.a = 5
    return vl
}

let vl1 = VeryLarge()
let vl2 = bar(vl1)

Will vl2.subStructB be a copy of vl1.subStructB, or an entirely new object?

I’m worried about performance when making small changes to large objects.

Thanks,
Daniel T.
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