PHP和类：访问父类中父项的公共属性

Question

here is what my code looks like

i have two forms:

class Form_1 extends Form_Abstract {

    public $iId = 1;

}
class Form_2 extends Form_1 {

    public $iId = 2;

}

i expect the code behave like this:

$oForm = new Form_2;
echo $oForm->getId(); // it returns '2'
echo $oForm->getParentId(); // i expect it returns '1'

here is my Form_Abstract class:

class Form_Abstract {

    public $iId = 0;

    public function getId() {
        return $this->iId;
    }

/**
this method will be called from a child instance
*/
    public function getParentId() {
        return parent::$iId;
    }
}

but it throws a Fatal Error:

Fatal error: Cannot access parent:: when current class scope has no parent

please help me with the method getParentId()

PS: i know the reason of what happens, i am seeking for the solution.

Answer 1

You have to use Reflection Api to access the parent class' property default value. Substitute getParentId, in Form_Abstract , with this, and all works fine:

public function getParentId() {
    $refclass = new ReflectionClass($this);
    $refparent = $refclass->getParentClass();
    $def_props = $refparent->getDefaultProperties();

    return $def_props['iId'];
}

Clearly you cannot call getParentId() in the root class, so it's better to check if a parent class exists.

UDATE:

You can do the same with classes/objects functions:

public function getParentId() {
    $def_values = get_class_vars(get_parent_class($this));
    return $def_values['iId'];
}

Answer 2

该错误是因为您正在调用没有父级的类的父级（它不扩展现有的类）。

Answer 3

I don't think it is even possible to access the "parent"'s version of $iId : you don't actually re-define it in the child class : you only chance the value that was defined in the parent's class.

To makes things very simple : when you declare the Form_2 class that extends Form_1 , it takes all the properties and methods of Form_2 , and put them in Form_1 , overriding what was already existing there.
There is no longer "two distinct classes" : there is only one resulting object, that's both Form_1 and Form_2 at the same time.

And here's an example that kind of -- I hope -- will help understand what I mean :

class Form_Abstract {}
class Form_1 extends Form_Abstract {
    public $iId = 1;
    public function methodInParent() {
        var_dump($this);
    }
}
class Form_2 extends Form_1 {
    public $iId = 2;
    public function tryingToGetParentProperty() {
        var_dump(parent::$iId); 
    }
}

$form2 = new Form_2();
$form2->methodInParent();
$form2->tryingToGetParentProperty();

Using this portion of code, the call to $form2->methodInParent() will get you :

object(Form_2)#1 (1) {
  ["iId"]=>
  int(2)
}

ie even if calling/executing a method that's defined in the parent's class, the $iId property is still the value defined in the child class : there is one, and only one, version of that property !

And the call to $form2->tryingToGetParentProperty() will get you :

Fatal error: Access to undeclared static property: Form_1::$iId

As there is no static property called $iId in Form_1 .

I suppose a solution to avoid that situation would be to declare $iId as static -- but note that it would change the meaning of your code, and the way it behaves !

ie the static variable will be shared accross all instances of the class -- which is probably not what you want ^^

Answer 4

i made it work by this:

public function getParentId() {
    $sClass = get_parent_class($this);
    $tmp = new $sClass;
    return $tmp->iId;
}

BUT is it a standard solution, does it have any performance issue ?