PHP and ?? operator

Question

Like some people know, C# has very useful ?? operator which evaluates and returns expression on the right if expression on the left is null. It's very useful for providing default values, for example:

int spaces = readSetting("spaces") ?? 5;

If readSetting couldn't find "spaces" and returns null, variable spaces would hold default value of 5.

You could do almost the same in JavaScript and Ruby with || operator, as in

var spaces = readSetting("spaces") || 5;

although you couldn't have 0 as value of spaces in JavaScript in this case and false in both Ruby and JavaScript.

PHP has or operator and although it doesn't work as || in a sense that it doesn't return expression from the right, it still could be useful here:

$spaces = readSetting('spaces') or $spaces = 5;

with the note that "" and "0" are also treated like false in PHP in addition to false, 0 and null in most languages.

The question is, should I use the construction from above? Does it have side effects apart from treating large class of characters as false? And is there better construction that is usually used and recommended by PHP community this task?

Answer 1

If you want to guarantee that you get false or null, and not treat things like "0" as "false", you could do the following:

$spaces = readSetting('spaces');
if($spaces == null || $spaces === false) $spaces = 5;

The === operator in PHP looks for an identical match, so "0", "", won't equate to false. Obviously you might want to restyle the code to your liking, but you get the idea.

Answer 2

It is a good idea to be more explicit in cases like this, especially in PHP since it has somewhat confusing type conversion rules (e.g. "0" is false as you pointed out).

If you want to be strict, let your function readSettings return the correct setting or a well-defined value if no setting is found, e.g. null. Then you should use it as:

$spaces = readSettings('spaces');
if (null === $spaces) {
    $spaces = 5;
}

If you want to be more generous and only want $spaces to be non-empty, use:

$spaces = readSettings('spaces');
if (empty($spaces)) {    // or:  if (!$spaces) {
    $spaces = 5;
}

You could shorten it by the cost of a second function call or ugly constructs (not recommended):

$spaces = readSettings('space') ? readSettings('space') : 5;
$spaces = ($x = readSettings('space')) ? $x : 5;  // UGLY!

But watch out if you want 0 to be a valid value for $spaces!

From the Zen of Python:

Explicit is better than implicit.

In your case I would recommend to just add a second paramter $default to readSettings():

function readSettings($key, $default=null) {
    return isset($settings[$key]) ? $settings[$key] : $default;
}

$spaces = readSettings('spaces', 5);

Answer 3

How about this?

$spaces = ($tmp=readSettings('space')) ? $tmp : 5;

Or, more confusing but using only one variable:

$spaces = ($spaces=readSettings('space')) ? $spaces : 5;

Answer 4

PHP 5.3.0 gives a shorter version of the ternary operator condition?true:false which goes as follows:

$spaces = readSettings('spaces') ?: 5;

Please notice that PHP 5.3.0 is still in beta and not yet production ready (its a release candidate already though), but it gives alot of new cool stuff like lambda functions and namespaces too, so it's definitely worth to check the features out!

Here is an article that describes the new features pretty well:

http://www.sitepoint.com/article/whats-new-php-5-3/