Structs and value semantics

A student record belongs together: a name, a score and operations on that record. A struct defines a new value type containing properties and methods. Assigning a struct produces another logical value, so changing a copy does not change the original record.

Properties describe stored data. Methods are functions attached to the type. A method that changes a struct’s own stored state must be marked mutating, and the instance being changed must be mutable. A simple struct receives a memberwise initializer when its declarations allow it.

Save this complete program as main.swift and run it with the Swift toolchain. No separate input file is needed.

Swift · complete program

struct Student {
  let name: String
  var score: Int
  mutating func addBonus() { score += 5 }
}
let original = Student(name: "Diya", score: 70)
var copy = original
copy.addBonus()
print(original.score, copy.score)

Expected output

70 75

Follow the execution

The original score stays 70, while copy becomes 75. The method changes only the mutable instance on which it was called. This observable independence is value semantics, regardless of how storage is optimized internally.

Try this program in the Swift compiler. Paste the complete source above.

Your task: Can original.addBonus() compile while original is let?

Swift · complete program

struct Student {
  let name: String
  var score: Int
  mutating func addBonus() { score += 5 }
}
let original = Student(name: "Diya", score: 70)
var copy = original
copy.addBonus()
print(original.score, copy.score)

Worked answer

No. The mutating method changes the struct value, and a let-bound struct cannot be mutated. Keep the original constant or declare a mutable instance for an intentional update.

A struct can contain references to class instances, so copying the struct does not necessarily clone every object reachable from it. Distinguish value semantics of the wrapper from reference semantics of its members.

No. The mutating method changes the struct value, and a let-bound struct cannot be mutated. Keep the original constant or declare a mutable instance for an intentional update. A struct can contain references to class instances, so copying the struct does not necessarily clone every object reachable from it.

Further reading: official Swift language guide.