How to create child object from existing parent object

Question

I have following php class

class User implements IUser
{
    public function __construct($i_objParent = NULL)
    {

    }
}

When I login successfully I create object of User class.

There is another class named Student class as follows which extends User Class

class Student extends User
{
    public function __construct($i_objParent = NULL)
    {
        parent::construct($i_objParent);
    }
}

Now as I said eariler I already have object of User class how can I construct Student object from existing User Class.

I think it may be possible by passing existing User class object to constructor of child class here Student class object?

Also, Is above approach OK?

write your corresponding constructor which expects a user object and copy its values — Fender, Aug 11 '11 at 12:35
@Fender - I am upto this stage but I am confused how to proceed? — Bharat Patil, Aug 11 '11 at 12:43
i am not sure if you can do it automatically. maybe you should assign the value of the fields from `$i_objParent` to the local ones: `$this->myField = $i_objParent->myField;` but i hope that there is a better way to do it — Fender, Aug 11 '11 at 12:47

score 3 · Accepted Answer · answered Aug 11 '11 at 12:54

As you cannot cast objects in php ( well there are really ugly hacks that work, but i would avoid them ).

Instead there are two ways:

create a factory class which takes the User instance in returns you new Student instance with all the data transfered
use Decorator pattern , which would call methods on User instance

Actually there is third way ( one that i would use ) : do not do this. PHP is not the language for DCI development paradigm. IMHO this whole construction makes no sense. User and Student are not interchangeable.

maybe this explains : http://stackoverflow.com/questions/505641/the-decorator-design-pattern/505658#505658 — tereško, Aug 11 '11 at 13:00

score 0 · Answer 2 · edited May 23 '17 at 12:05

It's best to have all classes either abstract or leaf, to avoid such problems. Inheritance cannot be applied to this design, You should change it to either

1 - Using User as a property of Student not as it's parent (which may end up in using Decorator pattern) 2 - Defining User as an abstract class and creating a Student object from the beginning

As you know polymorphic works upside down, and even object casting would not help you in this case.

Here is the same problem on java

score 0 · Answer 3 · answered Aug 11 '11 at 12:53

One class extending another does not have anything to do with parent / child relationship between them. It's a architecture solution, that allows you to re-use functionality, save on duplicating the code. I am not aware of any automatic call that would get the object that extends User class. I would create a function

public function getChild($type)
{
    switch($type)
    {
         case "user":
         return new Student();
         break;
         case "teacher":
         return new Teacher();
         break;
    }
}

How to create child object from existing parent object

3 Answers3