접근 제어(public/internal/fileprivate/private)

struct Greeter {            // internal by default
    func hello(_ name: String) -> String { "Hello, \\(name)" }
}
let g = Greeter()
print(g.hello("Ada"))

public struct Math {
    public static func add(_ a: Int, _ b: Int) -> Int { a + b }
    // internal helper remains hidden outside the module
    static func double(_ x: Int) -> Int { x * 2 }
}
print(Math.add(1,2))

// In FileA.swift
fileprivate func hiddenInFile() { print("Only in this file") }

struct A {
    func call() { hiddenInFile() } // OK: same file
}
A().call()

struct Box {
    private var value = 0     // visible only inside Box declaration
    mutating func set(_ v: Int) { value = v } // controlled write
    func get() -> Int { value }                // controlled read
}
var b = Box()
b.set(7)
print(b.get())
// b.value = 3   // error: 'value' is private