Answering the question from the perspective of value types vs reference types, from this Apple blog postthis Apple blog post it would appear very simple:
Use a value type [e.g. struct, enum] when:
- Comparing instance data with == makes sense
- You want copies to have independent state
- The data will be used in code across multiple threads
Use a reference type [e.g. class] when:
- Comparing instance identity with === makes sense
- You want to create shared, mutable state
As mentioned in that article, a class with no writeable properties will behave identically with a struct, with (I will add) one caveat: structs are best for thread-safe models -- an increasingly imminent requirement in modern app architecture.
