子类不从父类继承属性

Question

This isn't working the way I thought it would. Here are my classes:

class App {
    public $db;

    public function __construct($db) {
        $this->db = $db;
    }
}

class Analysis extends App {

    public $analysis_id;

    public function __construct($analysis_id) {
        $this->analysis_id = $analysis_id;
    }
}

class Standard extends Analysis {

    public function __construct($analysis_id) {
        parent::__construct($analysis_id);
    }
}

$db is my database (mysqli) object that I've passed to the App class.

When I try to perform a new Standard Analysis, I initiate it like this:

$analysis = new Standard($analysis_id);

The Analysis class contains methods that retrieve meta data about an analysis, while the Standard class contains methods that retrieve calculations for that specific type of analysis. I thought I would be able to access the $db object, but I can't from the Analysis or Standard class. Do I need to pass the $db object to the Standard class when I initiate it?

Answer 1

Your Analysis class needs to call the constructor on its parent, App. This doesn't happen by default. Because $db is a parameter of the App constructor, you will have to pass this in from the subclasses as well, and then call parent::__construct($db) from the Analysis constructor.

Correct code:

class App {
    public $db;

    public function __construct($db) {
        $this->db = $db;
    }
}

class Analysis extends App {
    public $analysis_id;

    public function __construct($analysis_id, $db) {
        $this->analysis_id = $analysis_id;
        parent::__construct($db);
    }
}

class Standard extends Analysis {
    public function __construct($analysis_id, $db) {
        parent::__construct($analysis_id, $db);
    }
}

$analysis = new Standard($analysis_id, $db);