Dynamic creation of objects

Firkrag

I need to create an object having its name and values passed to constructor in variables.

<?php
class Dod
{
	public $a;
	public $b;
	function __construct($aN,$bN)
	{
		$this->a=$aN;
		$this->b=$bN;
	}

}
class Pop
{
	public $z;
}

// $obj1=new Dod('a','b');

$class='Dod';
$arguments=array('a','b');

/* here I need code to create $obj2, 
its class name should be taken from $class 
and values passed to constructor from $arguments, 
so it should become the same as $obj1. */

/* The same code should also work for following data: */

$class='Pop';
$arguments=array();

?>

alimadzi

The first part is easy, but I'm not sure about passing an array of parameters instead of individual variables to the constructor...

$obj2 = new $class( /* something */ );

Firkrag

Simply passing an array of parameters doesn't work

laserlight

How does it not work? Show an example.

Firkrag

$class='Dod'; 
$arguments=array('a','b'); 
$obj2 = new $class( $arguments );

Missing argument 2 for Dod::__construct()

laserlight

That's because your constructor is written to take two variables, not a single array variable.

Firkrag

I know what the problem is, I just need a solution. I can't change the constructor.

laserlight

I can't change the constructor.

Oh, so it is not your constructor. In that case, you have to access the elements individually, e.g.,

$class = 'Dod';
$arguments = array('a','b');
$obj2 = new $class($arguments[0], $arguments[1]);

alimadzi

Ah (insert flash of inspiration here) I think I've got it...

$class = 'Dod';
$arguments = array( 'a', 'b' );

list( $arg1, $arg2 ) = $arguments;
$obj2 = new $class( $arg1, $arg2 );

I haven't tested this, but I think it should work...

alimadzi

Laserlight beat me to it by three minutes... and has a better solution anyway.

Firkrag

Both solutions are missing one point. This code will work, but only for constructors with two parameters. What about others like class 'Pop' that I mentioned at the beginning, and about many others classes that I don't yet know about?

suntra

Can you use eval() ? If yes, then your problem will be quickly solved 😉

laserlight

Both solutions are missing one point. This code will work, but only for constructors with two parameters. What about others like class 'Pop' that I mentioned at the beginning, and about many others classes that I don't yet know about?

What exactly are you trying to do? Generally, how a class is constructed is up to the class designer. Blindly trying to pass arguments from an array to an arbitrary constructor is rather unusual. If you have control over the class interface designs, then you can mandate that each class provide a constructor that takes an array, or some other fixed number of arguments. If you do not have control over the class interface designs, then you have to follow the rules set out in each class interface. Anything else would be making a mockery of the interface provided.

Firkrag

Using eval will work, but I was hoping there is more natural way of achieving the result.

Lets suppose I need to connect to the database. I have a class DB that does it. So every time I need a db connection somewhere i would have to write
new DB(x,y,z)
Instead I keep x,y,z in an array in a config file. The same applies to every class which constructor parameters can be written to a config file before script runs. Then I could request DB object from a part of script which would automatically get DB connection parameters from config, and use them to create an object. When I want an object of another class, then the same part of script would find appropriate parameters in config and use them to create desired object.

I tried to do similar thing with singletons but I don't want to make any changes in classes.

laserlight

Consider: that you have x, y, z implies that you know how to instantiate a DB object. As such, you could create a function wrapper that creates the object and returns it. You surely have a finite number of tasks that need such services, so you could implement a function wrapper for each of them.

Your main script can then just call the appropriate function wrapper to effectively "find appropriate parameters in config and use them to create desired object".

Firkrag

That is a good idea, but involves repetition of similar tasks, which I am trying to avoid. As of now I dont't see any disadventages of my idea + eval (except for using slow eval of course, and some possible mockery of interfaces🙂 ), so I will try to use it.

suntra

If you're using PHP5, you can use this :

http://ca3.php.net/call_user_func_array#74427 wrote:
If you are thinking of using call_user_func_array to instantiate an object (see comments below using Reflection) then since v5.1.3 you can use the Reflection::newInstanceArgs() method.
<?php

// arguments you wish to pass to constructor of new object
$args = array('a', 'b');

// class name of new object
$className = 'myCommand';

// make a reflection object
$reflectionObj = new ReflectionClass($className);

// use Reflection to create a new instance, using the $args
$command = $reflectionObj->newInstanceArgs($args);

// this is the same as: new myCommand('a', 'b', 'c');
?>