PHP: Passthrough unset arguments, not duplicating defaults

Question

class BetterDOMDocument extends DOMDocument 
{
    function __construct($version = null, $encoding = null) {
        parent::__construct($version, $encoding);
    }
}

This code will generate an XML header with an empty version attribute.

However, I cannot just define the constructor as:

function __construct($version, $encoding) {}

Because now PHP complains about Undefined Variable usage if I want to use this constructor without passing any arguments.

DOMDocument does not require be to passed $version, it'll use a default if I don't pass a value. How can I override the constructor while keeping the argument handling of the base class unchanged, without duplicating the specific default values?

I'm interested in this not in terms of a problem I'm stuck at that needs a workaround, but in terms of PHP language design. Can one , and how so, reproduce the behavior of DOMDocument in PHP, or is there something special about how internal code is able to deal with undefined values?

Your class constructor has the same arguments than the base class. What's your exact problem? Can you post a code snippet that reproduces the issue? — Álvaro González, Mar 12 '12 at 17:24

Wesley Murch · Answer 1 · 2012-03-14T10:01:54.567

3

Just set some sensible defaults:

class BetterDOMDocument extends DOMDocument 
{
    function __construct($version = '1.0', $encoding = 'UTF-8') {
        parent::__construct($version, $encoding);
    }
}

If this is actually all you're doing in the constructor, you don't need to overload it in the first place unless the purpose is set your own defaults.

In PHP, and I suspect in other languages, you actually can't tell a function to "use default" unless passing NULL as an argument (for instance) is handled by the class in such a way that it will use a default value instead. You'll have to work with tte design of the existing class/function, which may vary.

edited Mar 14 '12 at 10:01

answered Mar 12 '12 at 17:38

Wesley Murch

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Actually, ``DOMDocument`` does not require be to passed ``$version`` it'll use a default if I don't pass a value. The question is how I can reproduce the behavior without duplicating the actual default. I've updated the question to make this clearer. – miracle2k Mar 14 '12 at 09:53
Regarding your edit: My mistake, I read the [docs](http://www.php.net/manual/en/domdocument.construct.php) incorrectly. You actually can't tell a function to "use default" unless passing NULL as an argument (for instance) is handled by the class in such a way that it will use a default value instead. For example: http://stackoverflow.com/questions/9541776/php-function-ignore-some-default-parameters/9541834 – Wesley Murch Mar 14 '12 at 09:59
Also see: http://www.php.net/manual/en/functions.arguments.php There *may* be workarounds with stuff like `func_get_args()` for *some* instances, but it really should be dealt with on a case-by-case basis. The literal thing you're asking cannot be done. – Wesley Murch Mar 14 '12 at 10:07

PHP: Passthrough unset arguments, not duplicating defaults

1 Answers1