Object

Junior

Essence

An object is an instance of a class living on the heap: it has state (property values), behaviour (methods) and identity — a memory address that distinguishes it from another object holding the same data. In Swift an object in the strict sense is an instance of a class (or actor); an instance of a struct/enum is called a value — it has no identity, only equality.

Object vs value

final class Counter {
    var value = 0
    func increment() { value += 1 }
}

let a = Counter()
let b = a                 // a second reference to the same entity
b.increment()
print(a.value)            // 1
print(a === b)            // true — one identity

struct Point: Equatable { var x = 0 }
let p1 = Point()
var p2 = p1               // an independent copy
p2.x = 5
print(p1 == p2)           // false — values have equality but no identity

• === compares identity (the same object), == compares equality via Equatable. === does not exist for structs.
• An object can be mutated through any reference, even when the reference itself is a let. For a struct, let freezes the whole value.
• An object lives while strong references to it exist (ARC); deinit runs when the count hits zero. A value dies with its scope.

What else matters

• Messages: in Objective-C a method call is a message resolved at run time. In Swift a call is usually resolved by the compiler (static or vtable); messaging happens only through @objc dynamic.
• Inheritance, encapsulation and polymorphism are properties of classes, not objects; an object merely "carries" its class's behaviour.
• ObjectIdentifier(obj) exposes identity as a Hashable value — handy for dictionary keys and a Set of objects.

What the interviewer looks for

Can you explain when identity is needed (a shared mutable resource: a connection, a cache, a controller) and when a value is enough (a data model, a DTO, coordinates). Apple's answer: default to struct, use class when you need identity, inheritance or Objective-C interoperability.