What is the difference between let and var, and between structs and classes?
Assesses fundamental understanding of Swift conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
let declares a constant binding and var a mutable one. For value types, let also prevents changing stored properties.
Structs are value types: assignment and passing make a copy, with copy-on-write for collections. Classes are reference types: assignment shares the same instance and mutations are visible to every reference.
struct Point { var x = 0 }
class Box { var value = 0 }
var p = Point()
var p2 = p
p2.x = 1 // p.x stays 0
let b = Box()
let b2 = b
b2.value = 1 // b.value is 1, same object
Classes add inheritance, deinitialisers and identity comparison with ===. Structs get memberwise initialisers and avoid shared mutable state, which is safer under concurrency. The standard library and SwiftUI favour structs. Prefer structs unless you need reference semantics or inheritance.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.