Value & Reference Semantics
Core Idea
The important distinction is not simply where something is stored in memory.
The key question is:
What happens when a value is copied?
Value Semantics
A copy represents an independent value.
Common examples:
- struct
- enum
- Array
- Dictionary
- Set
- String
Conceptually:
a
↓ copy
b
a and b represent independent values
Reference Semantics
A copy can refer to the same underlying instance.
Example:
let a = MyClass()
let b = a
Conceptually:
a ──┐
├──> same instance
b ──┘
Important
Do not use:
struct = stack
class = heap
as the primary model.
Storage and semantics are different layers.
Copy-on-Write
Some Swift value types can provide value semantics while sharing storage internally until mutation occurs.
Common examples:
-
Array
-
Dictionary
-
Set
-
String
