Correct Implementation of Virtual Functions in PHP?

php oop polymorphism virtual-functions
erikbwork picture erikbwork · Aug 29, 2009 · Viewed 27.4k times · Source

at my working place (php only) we have a base class for database abstraction. When you want to add a new database table to the base layer, you have to create a subclass of this base class and override some methods to define individual behaviour for using this table. The normal behaviour should stay the same.

Now I have seen many new programmers at our company, who just override the method for the default behaviour. Some are so "nice" to put in all the default behaviour and just add there individual stuff where they like it, others kill themself trying to use the baseclass and their inheritor.

My first thought to solve this problem, was thinking about abstract methods that should be overriden by inheriting classes. But beside other arguments against abstract methods, "abstract" just does not show why the baseclass can't be used by its own and why these function should be overriden.

After some googling around I didn't find a good answer to implementing "real" virtual functions in php (just that there is a virtual function, that nearly kills all hope of a concrete implementation).

So, what would you do with this matter?

Answer

Bob Fanger picture Bob Fanger · Sep 1, 2009

In PHP all public and protected functions are "virtual". You can prevent functions from being overriden by prepending the final keyword. (Or by making them private, but this is probably a bad idea).

In the design of the baseclass I would think of behaviors that subclasses would want to affect. I would for example create empty functions like before_update() and after_insert().

function after_insert() {
  // Virtual
}

Which the baseclass will call when an update/insert event occurs.

Maybe an is_valid() function which always returns true in the baseclass, and use the commentblock to describe what the consequences are when a subclass return false.

Hopefully this would give you some inspiration.

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