When to call super

As you may notice, exactly when you call super can have an important effect on your overridden method.

Suppose you have an alternate implementation of recordGrade(_:) that recalculates the failedClasses each time a grade is recorded:

override func recordGrade(_ grade: Grade) { var newFailedClasses: [Grade] = []

for grade in grades {

if grade.letter == "F" { newFailedClasses.append(grade)

}

}

failedClasses = newFailedClasses

super.recordGrade(grade)

}

This version of recordGrade(_:) uses the grades array to find the current list of failed classes. If you’ve spotted a bug in the code above, good job! Because you call super last, if the new grade.letter is an F, the code won’t update the failedClasses array properly.

While it’s not a hard rule, it’s generally best practice to call the super version of a method first when overriding. That way, the superclass won’t experience any side effects introduced by its subclass, and the subclass won’t need to know the superclass’s implementation details.