Generics

Generic Structs

A generic type stores the same shape of data for different value types.

Store a typed value

storedValue
generic_structs.swift
Replay: real traced execution (multi-file project)
struct Box<Value> {
    let value: Value

    func label() -> String {
        return "box=\(value)"
    }
}

let storedValue = 10
let box = Box(value: storedValue)
let message = box.label()

print(message)

struct Box<Value> {
    let value: Value

    func label() -> String {
        return "box=\(value)"
    }
}

let storedValue = 4
let box = Box(value: storedValue)
let message = box.label()

print(message)

struct Box<Value> {
    let value: Value

    func label() -> String {
        return "box=\(value)"
    }
}

let storedValue = 22
let box = Box(value: storedValue)
let message = box.label()

print(message)

  1. storedValue ← 10, box ← Box<Int>(value: 10)

    9let storedValue→ 10 = 10  //@storedValue=4, 2210let box→ Box<Int>(value: 10) = Box(value: storedValue10)11let message = boxBox<Int>(value: 10).label()

  2. func label() -> String

    4func label() -> String {5    return "box=\(value10)"6}

  3. message ← box=10

    10let box = Box(value: storedValue)11let message→ box=10 = boxBox<Int>(value: 10).label()1213print(messagebox=10)
    outputbox=10

  1. storedValue ← 4, box ← Box<Int>(value: 4)

    9let storedValue→ 4 = 410let box→ Box<Int>(value: 4) = Box(value: storedValue4)11let message = boxBox<Int>(value: 4).label()

  2. func label() -> String

    4func label() -> String {5    return "box=\(value4)"6}

  3. message ← box=4

    10let box = Box(value: storedValue)11let message→ box=4 = boxBox<Int>(value: 4).label()1213print(messagebox=4)
    outputbox=4

  1. storedValue ← 22, box ← Box<Int>(value: 22)

    9let storedValue→ 22 = 2210let box→ Box<Int>(value: 22) = Box(value: storedValue22)11let message = boxBox<Int>(value: 22).label()

  2. func label() -> String

    4func label() -> String {5    return "box=\(value22)"6}

  3. message ← box=22

    10let box = Box(value: storedValue)11let message→ box=22 = boxBox<Int>(value: 22).label()1213print(messagebox=22)
    outputbox=22
generic type The type argument is chosen when the generic type is used, so each instance keeps a concrete value type.