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What’s the difference between a static variable and a class variable?

Swift version: 5.1

Paul Hudson    @twostraws   

Both the static and class keywords allow us to attach variables to a class rather than to instances of a class. For example, you might create a Student class with properties such as name and age, then create a static numberOfStudents property that is owned by the Student class itself rather than individual instances.

Where static and class differ is how they support inheritance: When you make a static property it becomes owned by the class and cannot be changed by subclasses, whereas when you use class it may be overridden if needed.

For example, here’s a Person class with one static property and one class property:

class Person {
    static var count: Int {
        return 250
    }

    class var averageAge: Double {
        return 30
    }
}

If we created a Student class by inheriting from Person, trying to override count (the static property) would fail to compile if uncommented, whereas trying to override averageAge (the class property) is OK:

class Student: Person {
    // THIS ISN'T ALLOWED
    // override static var count: Int {
    //    return 150
    // }

    // THIS IS ALLOWED
    override class var averageAge: Double {
        return 19.5
    }
}
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