What is the difference between final Class and Class?

What is the difference between final Class and Class?

final class A {
    
}

class B {    
    
}

Final is class modifier which prevents it from being inherited or being overridden. From apple documentation

> You can prevent a method, property, or subscript from being overridden > by marking it as final. Do this by writing the final modifier > before the method, property, or subscript’s introducer keyword (such > as final var, final func, final class func, and final subscript). > > Any attempt to override a final method, property, or subscript in a > subclass is reported as a compile-time error. Methods, properties, or > subscripts that you add to a class in an extension can also be marked > as final within the extension’s definition. > > You can mark an entire class as final by writing the final modifier > before the class keyword in its class definition (final class). Any > attempt to subclass a final class is reported as a compile-time error.

Classes marked with final can not be overridden.

---

Why should one care at all whether class can or can't be overridden?

There are two things to consider:

1. As an API designer/developer, you might have a class in your Framework that can be abused or misused when subclassed. final prevents class to be subclassed—mission accomplished. You can also use final to mark methods, properties, and even subscripts of non-final classes. This will have same effect, but for particular part of the class.
2. Marking class as final also tells Swift compiler that method should be called directly (static dispatch) rather than looking up a function from a method table (dynamic dispatch). This reduces function call overhead and gives you extra performance. You can read more on this on Swift Developer Blog.

> Use final when you know that a declaration does not need to be > overridden. The final keyword is a restriction on a class, method, or > property that indicates that the declaration cannot be overridden. > This allows the compiler to safely elide dynamic dispatch indirection.

Read more:

https://developer.apple.com/swift/blog/?id=27

Final means that no one can inherit from this class.

Besides final declaration does not need to be overridden also has better performance than no use final, because final disable dynamic dispatch at runtime which save runtime overhead.

Other answers are already given enough understanding about final keyword. I want to explain by some example.

Consider below example without final Keyword.

class A {
   public var name: String
   var breed: String
    
    init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}
class B:A{
    override init(name: String, breed: String) {
        super.init(name: name, breed: breed)
    }
}

In above code it allow to overwrite the varible of it's super class. While Class with final keyword don't allow it. Refer below example.

final class A {
   public var name: String
   var breed: String
    
    init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}

class B:A{
    **//ERROR:inheritance from a final class 'A' class B**
    override init(name: String, breed: String) {
        super.init(name: name, breed: breed)
    }
}

> Above code will give error inheritance from a final class A class B

Hoping I can boil down the other answers into this SSCCE that helped me:

          open class Open {}     // Anyone can see, anything can subclass
public         class Normal {}   // Anyone can see, internal can subclass
internal       class Internal {} // Internal can see, internal can subclass
public   final class Final {}    // Anyone can see, nothing can subclass

In your project/module:

class SubOpen:     Open {}     // OK
class SubNormal:   Normal {}   // OK
class SubInternal: Internal {} // OK
class SubFinal:    Final {}    // Error: Can't subclass

In some other project/module:

class SubOpen:     Open {}     // OK
class SubNormal:   Normal {}   // Error: Can't subclass
class SubInternal: Internal {} // Error: `Internal` type not found
class SubFinal:    Final {}    // Error: Can't subclass