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

User · Answer

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:

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

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

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

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.

User · Answer

Open is an access level  was introduced to impose limitations on class inheritance on Swift    This means that the open access level can only be applied to classes and class members   In Classes  An open class can be subclassed in the module it is defined in and in modules that import the module in which the class is defined   In Class members  The same applies to class members  An open method can be overridden by subclasses in the module it is defined in and in modules that import the module in which the method is defined   THE NEED FOR THIS UPDATE  Some classes of libraries and frameworks are not designed to be subclassed and doing so may result in unexpected behavior  Native Apple library also won t allow overriding the same methods and classes    So after this addition they will apply public and private access levels accordingly   For more details have look at Apple Documentation on Access Control

User · Answer

open is only for another module for example  cocoa pods  or unit test  we can inherit or override

User · Answer

open come to play when dealing with multiple modules   open class is accessible and subclassable outside of the defining module  An open class member is accessible and overridable outside of the defining module

User · Answer

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

[swift] What is the 'open' keyword in Swift?

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