Access Control

Prerequisites: Structs, Classes and reference types

Access control decides which code can see and use a type, property, or method. It lets you hide implementation details, so they can be changed without breaking code that depends on them. This idea is called encapsulation.

The levels

Level	Visible to
`private`	The enclosing declaration (and its extensions in the same file)
`fileprivate`	Anything in the same file
`internal`	Anything in the same module — the default
`package`	Other modules in the same package (Swift 5.9+)
`public`	Any module that imports yours
`open`	Like `public`, and classes can be subclassed outside the module

A module is one unit of code that's built together — an app, or one library target in a Swift package.

Example

struct PasswordValidator {
    private let minimumLength = 8

    func isStrong(_ password: String) -> Bool {
        isLongEnough(password) && hasDigit(password)
    }

    private func isLongEnough(_ password: String) -> Bool {
        password.count >= minimumLength
    }

    private func hasDigit(_ password: String) -> Bool {
        password.contains { $0.isNumber }
    }
}

let validator = PasswordValidator()
print(validator.isStrong("abc12345"))
print(validator.isStrong("password"))

true
false

isStrong is the type's interface. The private helpers can be renamed or rewritten freely — nothing outside can depend on them. Calling validator.hasDigit("x") from outside is a compile error: 'hasDigit' is inaccessible due to 'private' protection level.

Read-only from outside

private(set) allows reading everywhere but writing only inside the type — you saw it in the Methods lesson:

struct Account {
    private(set) var balance = 0
}

In apps and libraries

In an app, the default internal is usually fine; use private for helpers. In a library others import, only types marked public are visible, so you choose exactly what the library promises to support.

Common mistakes

Making everything public "just in case". Every public member is a promise to keep it working.