note 124609 added to language.oop5.variance

I would like to explain why covariance and contravariance are important, and why they apply to return types and parameter types respectively, and not the other way around. Covariance is probably easiest to understand, and is directly related to the Liskov Substitution Principle. Using the above example, let's say that we receive an AnimalShelter object, and then we want to use it by invoking its adopt() method. We know that it returns an Animal object, and no matter what exactly that object is, i.e. whether it is a Cat or a Dog, we can treat them the same. Therefore, it is OK to specialize the return type: we know at least the common interface of any thing that can be returned, and we can treat all of those values in the same way. Contravariance is slightly more complicated. It is related very much to the practicality of increasing the flexibility of a method. Using the above example again, perhaps the "base" method eat() accepts a specific type of food; however, a _particular_ animal may want to support a _wider range_ of food types. Maybe it, like in the above example, adds functionality to the original method that allows it to consume _any_ kind of food, not just that meant for animals. The "base" method in Animal already implements the functionality allowing it to consume food specialized for animals. The overriding method in the Dog class can check if the parameter is of type AnimalFood, and simply invoke parent::eat($food). If the parameter is _not_ of the specialized type, it can perform additional or even completely different processing of that parameter - without breaking the original signature, because it _still_ handles the specialized type, but also more. That's why it is also related clo sely to the Liskov Substitution: consumers may still pass a specialized food type to the Animal without knowing exactly whether it is a Cat or Dog. ---- Server IP: 45.112.84.4 Probable Submitter: 185.85.3.168 (proxied: 46.11.240.3) ---- Manual Page -- http://php.net/manual/en/language.oop5.variance.php Edit -- https://master.php.net/note/edit/124609 Del: integrated -- https://master.php.net/note/delete/124609/integrated Del: useless -- https://master.php.net/note/delete/124609/useless Del: bad code -- https://master.php.net/note/delete/124609/bad+code Del: spam -- https://master.php.net/note/delete/124609/spam Del: non-english -- https://master.php.net/note/delete/124609/non-english Del: in docs -- https://master.php.net/note/delete/124609/in+docs Del: other reasons-- https://master.php.net/note/delete/124609 Reject -- https://master.php.net/note/reject/124609 Search -- https://master.php.net/manage/user-notes.php