structure vs class in swift language

Question

From Apple book "One of the most important differences between structures and classes is that structures are always copied when they are passed around in your code, but classes are passed by reference." Can anyone help me understand what that means? To me, classes and structs seem to be the same.

Accepted Answer

Here's an example with a `class`. Note how when the name is changed, the instance referenced by both variables is updated. `Bob` is now `Sue`, everywhere that `Bob` was ever referenced.

```
class SomeClass {
    var name: String
    init(name: String) {
        self.name = name
    }
}

var aClass = SomeClass(name: "Bob")
var bClass = aClass // aClass and bClass now reference the same instance!
bClass.name = "Sue"

println(aClass.name) // "Sue"
println(bClass.name) // "Sue"
```

And now with a `struct` we see that the values are copied and each variable keeps its own set of values. When we set the name to `Sue`, the `Bob` struct in `aStruct` does not get changed.

```
struct SomeStruct {
    var name: String
    init(name: String) {
        self.name = name
    }
}

var aStruct = SomeStruct(name: "Bob")
var bStruct = aStruct // aStruct and bStruct are two structs with the same value!
bStruct.name = "Sue"

println(aStruct.name) // "Bob"
println(bStruct.name) // "Sue"
```

So for representing a stateful complex entity, a `class` is awesome. But for values that are simply a measurement or bits of related data, a `struct` makes more sense so that you can easily copy them around and calculate with them or modify the values without fear of side effects.

structure vs class in swift language

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