Explain Swift memory, value semantics, and GCD
Company: Snapchat
Role: Software Engineer
Category: Software Engineering Fundamentals
Difficulty: hard
Interview Round: Onsite
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`?
```swift
let queue = DispatchQueue(label: "test")
queue.async {
print(1)
}
queue.sync {
print(2)
}
print(3)
```
### Snippet 2
What is the output order and why?
```swift
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?
Quick Answer: This question evaluates a candidate's understanding of Swift memory management, value versus reference semantics, copy-on-write behavior, ARC stack vs heap distinctions, ownership qualifiers (weak vs unowned), and Grand Central Dispatch concurrency and execution order.