Question

Disclaimer: This is not homework. Also, I’m describing a simplified situation so bear with my semi-psudo code.

I have what I feel is a pretty standard OOP situation, but I’m not sure the best way to handle it. Let’s say I have 3 classes: Base, A, and B. Class A extends Base and B extends A. The Base class’s constructor performs a database query that pulls various meta data defined in an array. See below:

Class Base {

    public $meta_data = array('col1','col2');

    function __construct() {
        $db->query("select ".join(',',$this->meta_data)." from table");
    }

}

Class A extends Base {

    public $meta_data = array('col3','col4');

    function _construct() {
        parent::__construct();
    }

}

Class B extends A {

    public $meta_data = array('col5','col6');

    function _construct() {
        parent::__construct();
    }

}

When I create a new B() object, I want the Base constructor’s sql query to look like this:

select col1, col2, col3, col4, col5, col6 from table;

And when I create a new Base() object, I want the sql query to look like this:

select col1, col2 from table;

Now…I technically know how to accomplish this (in possibly a crude way) but I’m curious what the proper way is without too much code replication and keeping in mind flexibility for the future (ie, adding a subclass of B). I basically need to merge the $meta_data arrays on each inheritance level all the way up to the constructor in the Base class.

Any ideas?

Answer 1

When you extend the class and define the new property you are overriding the values set in the parent class. I would wind up removing the $meta_data array from all but Base. Then in your extending classes create a function similar to:

After solving it in a very simple way I wound up deciding I would go with something like the following in Base:

protected function addToMetaData(array $meta_data) {
    $this->meta_data = array_merge($this->meta_data, $meta_data);
}

Then in your extending class constructors you just pass in an array of the values you want to add to Base::$meta_data.