Exact difference between self::__construct() and new self()

Question

Can you tell me the exact difference between return self::__construct() and return new self()?

It seems one can actually return a self::__construct() from a __construct() call when the object is created, returning the object itself as if the first __construct() was never even called.

Solution 1

This is best illustrated in code:

class MyClass {

   public $arg;

   public function __construct ($arg = NULL) {
     if ($arg !== NULL) $this->arg = $arg;
     return $this->arg;
   }

   public function returnThisConstruct () {
     return $this->__construct();
   }

   public function returnSelfConstruct () {
     return self::__construct();
   }

   public function returnNewSelf () {
     return new self();
   }

}

$obj = new MyClass('Hello!');
var_dump($obj);
/*
  object(MyClass)#1 (1) {
    ["arg"]=>
    string(6) "Hello!"
  }
*/

var_dump($obj->returnThisConstruct());
/*
  string(6) "Hello!"
*/

var_dump($obj->returnNewSelf());
/*
  object(MyClass)#2 (1) {
    ["arg"]=>
    NULL
  }
*/

var_dump($obj->returnSelfConstruct());
/*
  string(6) "Hello!"
*/

return self::__construct() returns the value returned by the objects __construct method. It also runs the code in the constructor again. When called from the classes __construct method itself, returning self::__construct() will actually return the constructed class itself as the the method would normally do.

return new self(); returns a new instance of the calling object's class.

Solution 2

I believe that new self() would create a new instance of the class while self::__construct ()only calls the classes __construct method.

Comments

[gen_Eric]

And __construct methods don't usually have return values.

[N.B.]

@Rocket - __construct always has a return value - the object created.

[DaveRandom]

@N.B. That's not the case, is it? Construct does not return the instance of the class, it is called during instantiation and returns NULL. In practice, at least.

[gen_Eric]

@N.B.: Not exactly. When doing new class, the return value of __construct is ignored, and the object is returned. But, when calling __construct like a normal function, it does just that. It acts like a function and returns whatever it returns. ideone.com/IZSi8

[netcoder]

__construct should always return void. It's the new operator that returns an instance of the class, not the constructor.

[netcoder]

It's also actually quite weird to call the constructor of an object after it has been constructed. Some languages doesn't even allow it. You probably just should separate the re-usable constructor logic in another method, e.g.: init().

[halfpastfour.am]

@netcoder One wouldn't normally call a constructor after the class has been constructed. It seems however possible to re-construct while the class is being constructed, something that may come in handy in the future.

[Mehdi Karamosly]

@DaveRandom I thought a php constructor is supposed to only return the object he creates, not a value.