Accessing Parent Class' property from child

问题:

See the following example (PHP)

class Parent
{ 
  protected $_property;
  protected $_anotherP;

  public function __construct($var)
  {
    $this->_property = $var;
    $this->someMethod();  #Sets $_anotherP
  }

  protected function someMethod()
  ...
}

class Child extends Parent
{
  protected $parent;

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

  private function myMethod()
  {
    return $this->parent->_anotherP;  #Note this line
  }
}

I am new to OOP and am a bit ignorant.

Here to access the parents property I am using an instance of that class, which seems wrong :S (no need of being i child then). Is there an easy way, so that i can sync the parent properties with the child properties and can directly access $this->anotherP without having to use $this->parent->anotherP ?

回答1:

As your Child class is extending your Parent class, every properties and methods that are either public or protected in the Parent class will be seen by the Child class as if they were defined in the Child class -- and the other way arround.

When the Child class extends the Parent class, it can be seen as "Child is a Parent" -- which means the Child has the properties of the Parent, unless it redefines those another way.

(BTW, note that "parent" is a reserved keyword, in PHP -- which means you can't name a class with that name)

Here's a quick example of a "parent" class :

class MyParent {
    protected $data;
    public function __construct() {
        $this->someMethodInTheParentClass();
    }
    protected function someMethodInTheParentClass() {
        $this->data = 123456;
    }
}

And it's "child" class :

class Child extends MyParent {
    public function __construct() {
        parent::__construct();
    }
    public function getData() {
        return $this->data; // will return the $data property 
                            // that's defined in the MyParent class
    }
}

That can be used this way :

$a = new Child();
var_dump($a->getData());

And you'll get as output :

int 123456

Which means the $data property, defined in the MyParent class, and initialized in a method of that same MyParent class, is accessible by the Child class as if it were its own.

To make things simple : as the Child "is a" MyParent, it doesn't need to keep a pointer to... itself ;-)

回答2:

This may save you a few hours of searching around.

Remember: Your Child class only inherits the properties DEFINED in the Parent class... So if you instantiate an object using Parent class and then populate it with data, then this data will NOT be available in your child class...

It's super obvious of course, but I'm guessing others may run into the same issue.

A super simple solution is not to extend anything, simply pass the $object of your parent class into your child class through a constructor. This way you have access to all the properties and methods of the object generated by parent class

Example

class child {

    public parentObject;

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

}

If your $parentObject has a public property $name, then you can access it inside the child class with a function like:

public function print_name() {
    echo $this->parentObject->name;
}