Access Control and Modules

Who can see what

Swift's access control works at the level of modules (an app target, a framework or a Swift package target) and source files. From most to least restrictive: private (visible within the enclosing declaration and its extensions in the same file), fileprivate (the whole file), internal (the default: anywhere in the same module), package (Swift 5.9+: other modules in the same Swift package), public (visible to other modules, but classes cannot be subclassed and methods cannot be overridden outside the module) and open (public plus subclassing and overriding from other modules). private(set) and similar forms restrict only the setter. Good API design keeps most things internal or private and exposes a small, deliberate public surface from frameworks and packages. Unit tests can reach internal symbols with @testable import, so you do not need to make things public just for testing. Splitting an app into local Swift packages with clear public APIs improves build times and enforces boundaries between features.

Access levels in a package

Only the public API is visible to other modules.

// Sources/Pricing/PriceCalculator.swift  (module: Pricing)
public struct PriceCalculator {
    public init(taxRate: Double = 0.18) { self.taxRate = taxRate }

    public func total(forPaise subtotal: Int) -> Int {
        subtotal + tax(on: subtotal)
    }

    let taxRate: Double                           // internal: invisible to other modules

    private func tax(on amount: Int) -> Int {      // private: only inside this type
        Int((Double(amount) * taxRate).rounded())
    }
}

open class PricingRule {                          // open: subclassable from other modules
    public init() {}
    open func adjust(_ paise: Int) -> Int { paise }
}

// App target
import Pricing
let calculator = PriceCalculator()
let payable = calculator.total(forPaise: 49_900)   // 58_882
// calculator.taxRate                           // error: internal to Pricing

// Tests
// @testable import Pricing   -> internal members become visible to tests

Rooms in a house

private is your bedroom, fileprivate the flat, internal the whole building, package the housing society, public the front gate that visitors may use, and open a door you also allow visitors to remodel.

त्वरित जाँच: What is the default access level in Swift?

• private
• internal
• public
• open