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

Hopefully to save some time for those who want something akin to protected methods:

As per other answers, swift now provides the 'private' modifier - which is defined file-wise rather than class-wise such as those in Java or C# for instance. This means that if you want protected methods, you can do it with swift private methods if they are in the same file

1. Create a base class to hold 'protected' methods (actually private)
2. Subclass this class to use the same methods
3. In other files you cannot access the base class methods, even when you subclass either

e.g. File 1:

class BaseClass {
    private func protectedMethod() {
        
    }
}

class SubClass : BaseClass {
    func publicMethod() {
        self.protectedMethod()  //this is ok as they are in same file
    }
}

File 2:

func test() {
    var a = BaseClass()
    a.protectedMethod() //ERROR


    var b = SubClass()
    b.protectedMethod() //ERROR
}

class SubClass2 : BaseClass {
    func publicMethod() {
        self.protectedMethod() //ERROR
    }
}

Answer 2

Swift 4 / Swift 5

As per mentioned in the Swift Documentation - Access Control, Swift has 5 Access Controls:

• open and public: can be accessed from their module's entities and any module's entities that imports the defining module.

• internal: can only be accessed from their module's entities. It is the default access level.

• fileprivate and private: can only be accessed in limited within a limited scope where you define them.

---

What is the difference between open and public?

open is the same as public in previous versions of Swift, they allow classes from other modules to use and inherit them, i.e: they can be subclassed from other modules. Also, they allow members from other modules to use and override them. The same logic goes for their modules.

public allow classes from other module to use them, but not to inherit them, i.e: they cannot be subclassed from other modules. Also, they allow members from other modules to use them, but NOT to override them. For their modules, they have the same open's logic (they allow classes to use and inherit them; They allow members to use and override them).

What is the difference between fileprivate and private?

fileprivate can be accessed from the their entire files.

private can only be accessed from their single declaration and to extensions of that declaration that are in the same file; For instance:

// Declaring "A" class that has the two types of "private" and "fileprivate":
class A {
    private var aPrivate: String?
    fileprivate var aFileprivate: String?

    func accessMySelf() {
        // this works fine
        self.aPrivate = ""
        self.aFileprivate = ""
    }
}

// Declaring "B" for checking the abiltiy of accessing "A" class:
class B {
    func accessA() {
        // create an instance of "A" class
        let aObject = A()

        // Error! this is NOT accessable...
        aObject.aPrivate = "I CANNOT set a value for it!"

        // this works fine
        aObject.aFileprivate = "I CAN set a value for it!"
    }
}

---

What are the differences between Swift 3 and Swift 4 Access Control?

As mentioned in the SE-0169 proposal, the only refinement has been added to Swift 4 is that the private access control scope has been expanded to be accessible from extensions of that declaration in the same file; For instance:

struct MyStruct {
    private let myMessage = "Hello World"
}

extension MyStruct {
    func printMyMessage() {
        print(myMessage)
        // In Swift 3, you will get a compile time error:
        // error: 'myMessage' is inaccessible due to 'private' protection level

        // In Swift 4 it should works fine!
    }
}

So, there is no need to declare myMessage as fileprivate to be accessible in the whole file.

Answer 3

For Swift 1-3:

No, it's not possible. There aren't any private/protected methods and variables at all.

Everything is public.

Update Since Swift 4, it's possible see other answers in this thread

Answer 4

Swift 3 and 4 brought a lot of change also for the access levels of variables and methods. Swift 3 and 4 now has 4 different access levels, where open/public access is the highest (least restrictive) access level and private access is the lowest (most restrictive) access level:

• private functions and members can only be accessed from within the scope of the entity itself (struct, class, …) and its extensions (in Swift 3 also the extensions were restricted)
• fileprivate functions and members can only be accessed from within the source file where they are declared.
• internal functions and members (which is the default, if you do not explicitly add an access level key word) can be accessed anywhere within the target where they are defined. Thats why the TestTarget doesn't have automatically access to all sources, they have to be marked as accessible in xCode's file inspector.
• open or public functions and members can be accessed from anywhere within the target and from any other context that imports the current target’s module.

Interesting:

Instead of marking every single method or member as "private", you can cover some methods (e.g. typically helper functions) in an extension of a class / struct and mark the whole extension as "Private".

class foo { }

private extension foo {
    func somePrivateHelperFunction01() { }
    func somePrivateHelperFunction02() { }
    func somePrivateHelperFunction03() { }
}

This can be a good idea, in order to get better maintainable code. And you can easily switch (e.g. for unit testing) to non-private by just changing one word.

Apple documentation

Answer 5

till swift 2.0 there were only three access level [Public, internal, private] but in swift 3.0 apple added two new access level which are [ Open, fileType ] so now in swift 3.0 there are 5 access level Here I want to clear the role of these two access level 1. Open: this is much similar to Public but the only difference is that the Public can access the subclass and override, and Open access level can not access that this image is taken from Medium website and this describe the difference between open and public access

Now to second new access level 2. filetype is bigger version of private or less access level than internal The fileType can access the extended part of the [class, struct, enum] and private can not access the extended part of code it can only access the lexical scope this image is taken from Medium website and this describe the difference between fileType and Private access level

Answer 6

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

---

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

Answer 7

Swift 3.0 provides five different access controls:

1. open
2. public
3. internal
4. fileprivate
5. private

Open access and public access enable entities to be used within any source file from their defining module, and also in a source file from another module that imports the defining module. You typically use open or public access when specifying the public interface to a framework.

Internal access enables entities to be used within any source file from their defining module, but not in any source file outside of that module. You typically use internal access when defining an app’s or a framework’s internal structure.

File-private access restricts the use of an entity to its own defining source file. Use file-private access to hide the implementation details of a specific piece of functionality when those details are used within an entire file.

Private access restricts the use of an entity to the enclosing declaration. Use private access to hide the implementation details of a specific piece of functionality when those details are used only within a single declaration.

Open access is the highest (least restrictive) access level and private access is the lowest (most restrictive) access level.

Default Access Levels

All entities in your code (with a few specific exceptions) have a default access level of internal if you do not specify an explicit access level yourself. As a result, in many cases you do not need to specify an explicit access level in your code.

The release note on the topic:

Classes declared as public can no longer be subclassed outside of their defining module, and methods declared as public can no longer be overridden outside of their defining module. To allow a class to be externally subclassed or a method to be externally overridden, declare them as open, which is a new access level beyond public. Imported Objective-C classes and methods are now all imported as open rather than public. Unit tests that import a module using an @testable import will still be allowed to subclass public or internal classes as well as override public or internal methods. (SE-0117)

More information & details : The Swift Programming Language (Access Control)

Answer 8

Access control mechanisms as introduced in Xcode 6:

Swift provides three different access levels for entities within your code. These access levels are relative to the source file in which an entity is defined, and also relative to the module that source file belongs to.

• Public access enables entities to be used within any source file from their defining module, and also in a source file from another module that imports the defining module. You typically use public access when specifying the public interface to a framework.
• Internal access enables entities to be used within any source file from their defining module, but not in any source file outside of that module. You typically use internal access when defining an app’s or a framework’s internal structure.
• Private access restricts the use of an entity to its own defining source file. Use private access to hide the implementation details of a specific piece of functionality.

Public access is the highest (least restrictive) access level and private access is the lowest (or most restrictive) access level.

Default accecss it internal, and does as such not need to be specified. Also note that the private specifier does not work on the class level, but on the source file level. This means that to get parts of a class really private you need to separate into a file of its own. This also introduces some interesting cases with regards to unit testing...

Another point to me made, which is commented upon in the link above, is that you can't 'upgrade' the access level. If you subclass something, you can restrict it more, but not the other way around.

This last bit also affects functions, tuples and surely other stuff in the way that if i.e. a function uses a private class, then it's not valid to have the function internal or public, as they might not have access to the private class. This results in a compiler warning, and you need to redeclare the function as a private function.

Answer 9

In Beta 6, the documentation states that there are three different access modifiers:

• Public
• Internal
• Private

And these three apply to Classes, Protocols, functions and properties.

public var somePublicVariable = 0
internal let someInternalConstant = 0
private func somePrivateFunction() {}

For more, check Access Control.

Answer 10

Now in beta 4, they've added access modifiers to Swift.

from Xcode 6 beta 4 realese notes:

Swift access control has three access levels:

• private entities can only be accessed from within the source file where they are defined.
• internal entities can be accessed anywhere within the target where they are defined.
• public entities can be accessed from anywhere within the target and from any other context that imports the current target’s module.

By default, most entities in a source file have internal access. This allows application developers to largely ignore access control while allowing framework developers full control over a framework's API.

Answer 11

As of Xcode 6 beta 4, Swift has access modifiers. From the release notes:

Swift access control has three access levels:

• private entities can only be accessed from within the source file where they are defined.
• internal entities can be accessed anywhere within the target where they are defined.
• public entities can be accessed from anywhere within the target and from any other context that imports the current target’s module.

The implicit default is internal, so within an application target you can leave access modifiers off except where you want to be more restrictive. In a framework target (e.g. if you're embedding a framework to share code between an app and an sharing or Today view extension), use public to designate API you want to expose to clients of your framework.

Answer 12

When one talks about making a "private method" in Swift or ObjC (or ruby or java or…) those methods aren't really private. There's no actual access control around them. Any language that offers even a little introspection lets developers get to those values from outside the class if they really want to.

So what we're really talking about here is a way to define a public-facing interface that merely presents the functionality we want it to, and "hides" the rest that we consider "private".

The Swift mechanism for declaring interfaces is the protocol, and it can be used for this purpose.

protocol MyClass {
  var publicProperty:Int {get set}
  func publicMethod(foo:String)->String
}

class MyClassImplementation : MyClass {
  var publicProperty:Int = 5
  var privateProperty:Int = 8

  func publicMethod(foo:String)->String{
    return privateMethod(foo)
  }

  func privateMethod(foo:String)->String{
    return "Hello \(foo)"
  }
}

Remember, protocols are first-class types and can be used anyplace a type can. And, when used this way, they only expose their own interfaces, not those of the implementing type.

Thus, as long as you use MyClass instead of MyClassImplementation in your parameter types, etc. it should all just work:

func breakingAndEntering(foo:MyClass)->String{
  return foo.privateMethod()
  //ERROR: 'MyClass' does not have a member named 'privateMethod'
}

There are some cases of direct assignment where you have to be explicit with type instead of relying on Swift to infer it, but that hardly seems a deal breaker:

var myClass:MyClass = MyClassImplementation()

Using protocols this way is semantic, reasonably concise, and to my eyes looks a lot like the Class Extentions we've been using for this purpose in ObjC.

Answer 13

One of the options you could use is to wrap the instance creation into a function and supply the appropriate getters and setters in a constructor:

class Counter {
    let inc: () -> Int
    let dec: () -> Int

    init(start: Int) {
        var n = start

        inc = { ++n }
        dec = { --n }
    }
}


let c = Counter(start: 10)

c.inc()  // 11
c.inc()  // 12
c.dec()  // 11

Answer 14

Using a combination of protocols, closures, and nested/inner classes, it's possible to use something along the lines of the module pattern to hide information in Swift right now. It's not super clean or nice to read but it does work.

Example:

protocol HuhThing {
  var huh: Int { get set }
}

func HuhMaker() -> HuhThing {
   class InnerHuh: HuhThing {
    var innerVal: Int = 0
    var huh: Int {
      get {
        return mysteriousMath(innerVal)
      }

      set {
       innerVal = newValue / 2
      }
    }

    func mysteriousMath(number: Int) -> Int {
      return number * 3 + 2
    }
  }

  return InnerHuh()
}

HuhMaker()
var h = HuhMaker()

h.huh      // 2
h.huh = 32 
h.huh      // 50
h.huh = 39
h.huh      // 59

innerVal and mysteriousMath are hidden here from outside use and attempting to dig your way into the object should result in an error.

I'm only part of the way through my reading of the Swift docs so if there's a flaw here please point it out, would love to know.

Answer 15

As far as I can tell, there are no keywords 'public', 'private' or 'protected'. This would suggest everything is public.

However Apple may be expecting people to use “protocols” (called interfaces by the rest of the world) and the factory design pattern to hide details of the implementation type.

This is often a good design pattern to use anyway; as it lets you change your implementation class hierarchy, while keeping the logical type system the same.

Answer 16

The language grammar does not have the keywords 'public', 'private' or 'protected'. This would suggest everything is public. Of course, there could be some alternative method of specifying access modifiers without those keywords but I couldn't find it in the language reference.