Abstract classes in Swift Language

There are no abstract classes in Swift (just like Objective-C). Your best bet is going to be to use a Protocol, which is like a Java Interface.

With Swift 2.0, you can then add method implementations and calculated property implementations using protocol extensions. Your only restrictions are that you can't provide member variables or constants and there is no dynamic dispatch.

An example of this technique would be:

protocol Employee {
    var annualSalary: Int {get}
}

extension Employee {
    var biweeklySalary: Int {
        return self.annualSalary / 26
    }

    func logSalary() {
        print("$\(self.annualSalary) per year or $\(self.biweeklySalary) biweekly")
    }
}

struct SoftwareEngineer: Employee {
    var annualSalary: Int

    func logSalary() {
        print("overridden")
    }
}

let sarah = SoftwareEngineer(annualSalary: 100000)
sarah.logSalary() // prints: overridden
(sarah as Employee).logSalary() // prints: $100000 per year or $3846 biweekly

Notice that this is providing "abstract class" like features even for structs, but classes can also implement the same protocol.

Also notice that every class or struct that implements the Employee protocol will have to declare the annualSalary property again.

Most importantly, notice that there is no dynamic dispatch. When logSalary is called on the instance that is stored as a SoftwareEngineer it calls the overridden version of the method. When logSalary is called on the instance after it has been cast to an Employee, it calls the original implementation (it doesn't not dynamically dispatch to the overridden version even though the instance is actually a Software Engineer.

For more information, check great WWDC video about that feature: Building Better Apps with Value Types in Swift

Note that this answer is targeted at Swift 2.0 and above

You can achieve the same behaviour with protocols and protocol extensions.

First, you write a protocol that acts as an interface for all the methods that have to be implemented in all types that conform to it.

protocol Drivable {
    var speed: Float { get set }
}

Then you can add default behaviour to all types that conform to it

extension Drivable {
    func accelerate(by: Float) {
        speed += by
    }
}

You can now create new types by implementing Drivable.

struct Car: Drivable {
    var speed: Float = 0.0
    init() {}
}

let c = Car()
c.accelerate(10)

So basically you get:

Compile time checks that guarantee that all Drivables implement speed
You can implement default-behaviour for all types that conform to Drivable (accelerate)
Drivable is guaranteed not to be instantiated since it's just a protocol

This model actually behaves much more like traits, meaning you can conform to multiple protocols and take on default implementations of any of them, whereas with an abstract superclass you're limited to a simple class hierarchy.

I think this is the closest to Java's abstract or C#'s abstract:

class AbstractClass {

    private init() {

    }
}

Note that, in order for the private modifiers to work, you must define this class in a separate Swift file.

EDIT: Still, this code doesn't allow to declare an abstract method and thus force its implementation.

The simplest way is to use a call to fatalError("Not Implemented") into the abstract method (not variable) on the protocol extension.

protocol MyInterface {
    func myMethod() -> String
}


extension MyInterface {

    func myMethod() -> String {
        fatalError("Not Implemented")
    }

}

class MyConcreteClass: MyInterface {

    func myMethod() -> String {
        return "The output"
    }

}

MyConcreteClass().myMethod()

Abstract classes in Swift Language

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