Explain Swift memory, value semantics, and GCD

Answer the following Swift/iOS fundamentals questions.

A) GCD execution order + threads

Assume the current thread is the main thread.

Snippet 1

What is the output order? Which thread executes each print?

let queue = DispatchQueue(label: "test")
queue.async {
    print(1)
}
queue.sync {
    print(2)
}
print(3)

Snippet 2

What is the output order and why?

var str = "1"
DispatchQueue.main.async {
    str += "2"
    print(str)
}
DispatchQueue.main.async {
    str += "3"
    print(str)
}
print(str)

Also explain why the second async block prints "123" rather than "13".

B) Swift/iOS conceptual questions

1. Compare class vs struct in Swift (semantics, performance, common use cases).
2. What is copy-on-write (CoW) in Swift? How would you implement CoW behavior for a custom type?
3. Explain stack vs heap in the context of Swift/ARC and what actually “goes where.”
4. Compare weak vs unowned . Why can unowned crash while weak typically doesn’t?