Does Swift have access modifiers?

Question

In Objective-C instance data can be public, protected or private. For example:

@interface Foo : NSObject
{
  @public
    int x;
  @protected:
    int y;
  @private:
    int z;
  }
-(int) apple;
-(int) pear;
-(int) banana;
@end

I haven't found any mention of access modifiers in the Swift reference. Is it possible to limit the visibility of data in Swift?

Answer 1

As of Swift 3.0.1, there are 4 levels of access, described below from the highest (least restrictive) to the lowest (most restrictive).

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1. open and public

Enable an entity to be used outside the defining module (target). You typically use open or public access when specifying the public interface to a framework.

However, open access applies only to classes and class members, and it differs from public access as follows:

• public classes and class members can only be subclassed and overridden within the defining module (target).
• open classes and class members can be subclassed and overridden both within and outside the defining module (target).

// First.framework – A.swift

open class A {}

// First.framework – B.swift

public class B: A {} // ok

// Second.framework – C.swift

import First

internal class C: A {} // ok

// Second.framework – D.swift

import First

internal class D: B {} // error: B cannot be subclassed

2. internal

Enables an entity to be used within the defining module (target). You typically use internal access when defining an app’s or a framework’s internal structure.

// First.framework – A.swift

internal struct A {}

// First.framework – B.swift

A() // ok

// Second.framework – C.swift

import First

A() // error: A is unavailable

3. fileprivate

Restricts the use of an entity to its defining source file. You typically use fileprivate access to hide the implementation details of a specific piece of functionality when those details are used within an entire file.

// First.framework – A.swift

internal struct A {

    fileprivate static let x: Int

}

A.x // ok

// First.framework – B.swift

A.x // error: x is not available

4. private

Restricts the use of an entity to its enclosing declaration. You typically use private access to hide the implementation details of a specific piece of functionality when those details are used only within a single declaration.

// First.framework – A.swift

internal struct A {

    private static let x: Int

    internal static func doSomethingWithX() {
        x // ok
    }

}

A.x // error: x is unavailable