Compare by reference?

[killingmoon]

Anyone know of a way to compare two objects/arrays by reference or address, rather than the standard by-value comparison?

I'm trying to check for recursive dependencies and of course if I have one, PHP dies with the error "Fatal error: Nesting level too deep - recursive dependency?". Any suggestions for other safe ways to check for recursive dependencies while traversing arrays would also be welcome.

I've done a bit of extension coding, so I know it would be fairly easy to write a function that compares the zvals directly. However, I really don't want to go the way of a custom extension just for this, mainly for portability reasons: I'm not even sure I can install a custom extension on the server I'm working on just now, nevermind all the messing about getting it compiled against the right source tree.

NB: I'm working with PHP 4.3

• Robert

[Weedpacket]

I've got the feeling that this isn't feasible in PHP4.3; the reason I think this is because in PHP5 $object1==$object2 compares objects' values, while $object1===$object2 checks to see if the two variables refer to the same object.

You could wiggle one variable by making some pissant change to its value (rather, a change to the value's elements or properties), and see if the change is reflected in the other variable (then change it back), but that sounds like a nice way to get into grief.

[killingmoon]

Weedpacket wrote:

You could wiggle one variable by making some pissant change to its value (rather, a change to the value's elements or properties), and see if the change is reflected in the other variable (then change it back), but that sounds like a nice way to get into grief.

I think you may be right about this being the only way. I wrote a function this morning to do it, hopefully safely. This wouldn't be thread-safe, but I don't think that matters in PHP4. I've posted it here for use by anyone else that wants it, please forward on any comments, suggested improvements, etc.

/**
 * is_equal
 *
 *     Check if two objects are identical ( like === operator ).
 *     Composite types (objects and arrays) are compared by reference instead of by value.
 *
 * LICENCE:
 *
 *     Free for any use provided the copyright notice is not removed from the source code.
 *     Provided "as-is" with no warranty, express or implied.
 *
 * @version    1.0
 * @author     Robert Tweed <[e-mail]>
 * @copyright  Copyright &copy; Robert Tweed, 2005
 * @link       http://www.killingmoon.com/
 *
 * @param   mixed    $a  First operand for comparison.
 * @param   mixed    $b  Second operand for comparison.
 * @return  boolean      True only if inputs are identical.
 *
 */

function is_equal( &$a, &$b ) {

//-- Make sure both operands are same type
$array = false;
if( is_array( $a ) )
	$array = true;

//-- One is an array, other must be an array too
if( $array ) {
	if( ! is_array( $b ) )
		return false;
}

//-- If either operand is a primitive type, do a simple comparison
//   otherwise both are objects
else if( ! is_object( $a ) || ! is_object( $b ) ) {
	return ( $a === $b );
}

//-- Generate a unique key that can be safely added to both operands
$key = '__TESTKEY__';
while(
	array_key_exists( $key, $a ) 
	|| array_key_exists( $key, $b )
	) {

	$key .= '_';
}

//-- Set the test key in first operand
if( $array )
	$a[ $key ] = true;
else
	$a->$key = true;

//-- Check if key now exists in second operand
if( array_key_exists( $key, $b ) )
	$eq = true;
else
	$eq = false;

//-- Remove key that was added earlier
if( $array )
	unset( $a[ $key ] );
else
	unset( $a->$key );		

//-- Return the result
return $eq;
}

• Robert