Question

Is there a reason why Magento has a _construct and a __construct method? Why does the additional _construct exist? Could anything achieved by having the extra _construct method not be achieved by just calling the parent constructor in the child class?

Answer 1

Best answer I can find: http://www.magentocommerce.com/boards/viewthread/76027/#t282659

Basically, the root-level class (from which all other classes inherit) implements __construct, which PHP calls automatically whenever a class is constructed. Right now, this root-level class simply calls _construct, which contains the actual code.

Say you have this set-up:

class BaseClass {
   function __construct() {
       print "In BaseClass constructor\n";
       doSomethingReallyImportant();
   }
}

class SubClass extends BaseClass {
   function __construct() {
       print "In SubClass constructor\n";
   }
}

$obj = new BaseClass();
//"In BaseClass constructor"
//something really important happens

$obj = new SubClass();
//"In SubClass constructor"
//important thing DOESN'T happen

PHP doesn’t automatically call the parent class constructors, so doSomethingReallyImportant never gets called. You could require that subclass constructors call parent::__construct(), but that’s easy to forget. So Magento has subclasses override _construct:

class BaseClass {
   function __construct() {
       doSomethingReallyImportant();
      _construct();
   }
   function _construct() {
       print "In BaseClass constructor\n";
   }
}

class SubClass extends BaseClass {
   function _construct() {
       print "In SubClass constructor\n";
   }
}

$obj = new BaseClass();
//something really important happens
//"In BaseClass constructor"

$obj = new SubClass();
//something really important happens
//"In SubClass constructor"

PHP doesn’t detect a constructor in SubClass, so it calls BaseClass‘s constructor. This allows BaseClass to doSomethingReallyImportant before calling SubClass’s overridden _construct.