What is the 'open' keyword in Swift?

Question

The ObjectiveC.swift file from the standard library contains the following few lines of code around line 228:

extension NSObject : Equatable, Hashable {   /// ...   open var hashValue: Int {     return hash   } } 

What does open var mean in this context, or what is the open keyword in general?

Answer 1

open is a new access level in Swift 3, introduced with the implementation of

• SE-0117 Allow distinguishing between public access and public overridability

It is available with the Swift 3 snapshot from August 7, 2016, and with Xcode 8 beta 6.

In short:

• An open class is accessible and subclassable outside of the defining module. An open class member is accessible and overridable outside of the defining module.
• A public class is accessible but not subclassable outside of the defining module. A public class member is accessible but not overridable outside of the defining module.

So open is what public used to be in previous Swift releases and the access of public has been restricted. Or, as Chris Lattner puts it in SE-0177: Allow distinguishing between public access and public overridability:

“open” is now simply “more public than public”, providing a very simple and clean model.

In your example, open var hashValue is a property which is accessible and can be overridden in NSObject subclasses.

For more examples and details, have a look at SE-0117.

Answer 2

Read open as

open for inheritance in other modules

I repeat open for inheritance in other modules. So an open class is open for subclassing in other modules that include the defining module. Open vars and functions are open for overriding in other modules. Its the least restrictive access level. It is as good as public access except that something that is public is closed for inheritance in other modules.

From Apple Docs:

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

1. Classes with public access, or any more restrictive access level, can be subclassed only within the module where they’re defined.

2. Class members with public access, or any more restrictive access level, can be overridden by subclasses only within the module where they’re defined.

3. Open classes can be subclassed within the module where they’re defined, and within any module that imports the module where they’re defined.

4. Open class members can be overridden by subclasses within the module where they’re defined, and within any module that imports the module where they’re defined.