PHP Inheritance - access parents properties

Question

Assume the following structure:

<?php
 class Event_Dates {

    public $start_date;

    public $end_date;   

    public function __construct( $start = null, $end = null ){
        $this->start_date = $start;
        $this->end_date = $end;
    }

    public function get_start_date(){
        return $this->start_date;
    }

    public function get_end_date(){
        return $this->end_date;
    }
}


class Event_Times extends Event_Dates
{
    public function __construct()
    {
        parent::__construct();
    }

    public function get_parent_start_date(){
        return $this->start_date;
    }

    public function get_parent_end_date(){
        return $this->end_date;
    }
 }
?>

Here's my client code:

<?php
 $start = "2017-03-12 04:00:00";
 $end = "2017-03-12 17:00:00";
 $event_dates = new Event_Dates( $start, $end );
 $event_times = new Event_Times(); 
?>

And the tests:

<?php 
 var_dump( $event_dates->get_start_date() ); // string(19) "2017-03-12 04:00:00" 
 var_dump( $event_dates->get_end_date() ); // string(19) "2017-03-12 17:00:00"
 var_dump( $event_times->get_parent_start_date() ); // NULL
 var_dump( $event_times->get_parent_end_date() ); // NULL
?>

As far as I can see, I'm using properties of Inheritance correctly. So why can't I access the parent class properties through my child class?

your parent::__construct(); makes no sense at the moment

RomanPerekhrest
– RomanPerekhrest

2017-03-18 20:35:31 +00:00
Commented Mar 18, 2017 at 20:35 — RomanPerekhrest
– RomanPerekhrest, Commented Mar 18, 2017 at 20:35

Mark Baker · Accepted Answer · 2017-03-18 20:38:43Z

1

You're misunderstanding inheritance completely, and confusing class definitions with instances.... each instance you create is unique, and has its own property values; but you don't create separate instances of each class within the inheritance tree, and expect them to access each other.

An instance of the child class inherits the properties and methods of the parent class

class Event_Dates {

    public $start_date;

    public $end_date;   

    public function __construct( $start = null, $end = null ){
        $this->start_date = $start;
        $this->end_date = $end;
    }

    public function get_start_date(){
        return $this->start_date;
    }

    public function get_end_date(){
        return $this->end_date;
    }
}


class Event_Times extends Event_Dates
{
    public function __construct( $start = null, $end = null )
    {
        parent::__construct( $start, $end );
    }

    public function get_parent_start_date(){
        return $this->start_date;
    }

    public function get_parent_end_date(){
        return $this->end_date;
    }
 }

$start = "2017-03-12 04:00:00";
$end = "2017-03-12 17:00:00";
$events = new Event_Times( $start, $end );


 var_dump( $events->get_start_date() ); // string(19) "2017-03-12 04:00:00" 
 var_dump( $events->get_end_date() ); // string(19) "2017-03-12 17:00:00"
 var_dump( $events->get_parent_start_date() ); string(19) "2017-03-12 04:00:00" 
 var_dump( $events->get_parent_end_date() ); string(19) "2017-03-12 17:00:00"

edited Mar 18, 2017 at 20:38

answered Mar 18, 2017 at 20:35

Mark Baker

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4 Comments

RomanPerekhrest Over a year ago

get_parent_start_date() and get_parent_end_date() don't give any benefit as Event_Times class is already inherited superclass getters. Such overridden is a waste

Mark Baker Over a year ago

@RomanPerekhrest - I know, but short of writing a full tutorial, I'm trying to emphasise the difference between classes and instances, as a way of explaining inheritance

cookie Over a year ago

@RomanPerekhrest - get_parent_start_date() & get_parent_end_date() were merely an illustration to get the point across. The point I was really trying to make was 'how do i get access to the parent properties'?

cookie Over a year ago

Great answers, but I have to select one. I feel @MarkBaker appeared to re-enforce OOP stuff somewhat.

lessan · Accepted Answer · 2017-03-18 20:41:27Z

you need to specify the $start_date and $construct_date when you construct the Event_Times too, otherwise it will call that method with the default values, which are null in you case

try

    class Event_Times extends Event_Dates
{
    private $parent;
    public function __construct($parent)
    {
        $this->start_date = $parent->start_date;
        $this->end_date = $parent->end_date;
        }

    public function get_parent_start_date(){
        return $this->start_date;
    }

    public function get_parent_end_date(){
        return $this->end_date;
    }
 }

then your code

    <?php
 $start = "2017-03-12 04:00:00";
 $end = "2017-03-12 17:00:00";
 $event_dates = new Event_Dates( $start, $end );
 $event_times = new Event_Times($event_dates); 
?>

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