Automatic Reference Counting (ARC)

How it works

Keeping the count of strong references
The instance is not deallocated until the last strong reference is broken

Retain Cycle (Strong Reference Cycle)

Problem: Memory Leak
Cause: two strong references pointing to each other
Solution:
- Weak references
- Unowned References
  - Alwasys refers to an instance that has not been deallocated
  - Runtime error occurs when access unowned reference after the instance has been deallocated
  - An unowned optional reference can be nil

Types that are managed by ARC (Reference Type)

Class
Closure

Types that are NOT managed by ARC (Value Type)

Basic types, like Bool, Int, etc.
Struct
Enum
Tuple