References, string offsets and overloaded objects

[Oleg]

Fatal error: Cannot create references to/from string offsets nor overloaded objects

I will give a cookie to whoever can explain:

1. What a "string offset" is; (is it a number?)

2. Why references to them cannot be created;

3. Why "overloaded objects" are a distinct class from "objects"; (I think I figured out what an "overloaded object" is)

4. Why references to "overloaded objects" cannot be created.

[igorw]

1. What a "string offset" is; (is it a number?)

$string = 'test';
echo $string[0]; // string offset 0 = t
echo $string[1]; // string offset 1 = e
echo $string[2]; // string offset 2 = s
echo $string[3]; // string offset 3 = t

It's basically the value of the string at a specific position.

2. Why references to them cannot be created;

I guess the implementation would be messy and not really useful. Referencing the string and reading the requested offset is possible as a workaround.


Code producing the error?

[ToonArmy]

String offset:

Code: Select all

php -r '$bar = "foo"; $o = &$bar[1];'

Fatal error: Cannot create references to/from string offsets nor overloaded objects in Command line code on line 1

$string[$x] is equivalent to substr($string, $x, 1)

I wonder if the overloaded error is a throwback to PHP4's overload extension.

[naderman]

You can reference zvals. A particular character inside a string does not have its own zval. PHP does not know a character type. Referencing such a character would mean that each character needs to be treated as a variable with reference counting etc. which would mean a lot of memory overhead. And overloading in PHP refers to special __get and __set methods. So you cannot create a reference to $foo->bar when bar is really being accessed through __get/__set rather than being a real public attribute of the class which would have a zval to reference.

[Oleg]

I thought "overloaded object" meant "object with at least one property overloaded". Is it a misname for "return value of an overloaded property" then? (because the latter is clearly not necessarily an object)

[naderman]

Yeah the error message is incorrect or at least misunderstandable.

[Oleg]

That's what I thought.

You can reference zvals.

This is the truth, and a case of major abstraction leakage.

I knew intuitively that references were broken for quite some time, now I understand why they are. It all makes sense now: passing by reference, returning references, reference bugs labeled as features, etc.

Even perl gets them right:

Code: Select all

% perl -e '$a = \4; print $$a."\n";'
4

And this is the original code.

Code: Select all

<?php

date_default_timezone_set('UTC');

define('IN_PHPBB', true);
$phpbb_root_path = (defined('PHPBB_ROOT_PATH')) ? PHPBB_ROOT_PATH : './';
$phpEx = 'php';
include($phpbb_root_path . 'common.' . $phpEx);

$user->date_format = '%b %m %Y | %b %m %Y';
$user->lang['datetime'] = '%b %m %Y';
$user->timezone = 'America/New_York';
print $user->format_date(time());

Here I am trying to test some timezone-related behavior, guessing what I need to do to bootstrap enough of phpbb so that format_date runs.

[ToonArmy]

You need to call the setup method on user.

[naderman]

You wouldn't want to call setup in a test scenario but just set up as little as necessary to get date_format to work. Generally references in PHP should be avoided, see http://schlueters.de/blog/archives/125- ... ences.html

[ToonArmy]

You wouldn't want to call setup in a test scenario but just set up as little as necessary to get date_format to work. Generally references in PHP should be avoided, see http://schlueters.de/blog/archives/125- ... ences.html

I wasn't aware this was unit testing related.