Function argument: default value return of a function?

Question

I have lots of functions with optional arguments, which on an omitted value gets its default value from a specified function, currently my code looks something like this:

function get_user($user_id = FALSE) {

   // If no ID is passed, get from session
   if(!$user_id) { 
      $user_id = get_id_from_session();      
   }

   // ... do something with the ID
}

It works fine, but it easily gets very clutty when having more then one optional argument. Instead, I'd prefer to do something like the following:

function get_user($user_id = get_id_from_session()) {

   // ... do something with the ID

}

I'm sure that you can see how that is more convenient. Is there any way to accomplish this, or do anyone have suggestions on another cleaner approach to do this?

nickb · Accepted Answer · 2012-08-06 17:29:36Z

4

The only way that you can "shorten" this is to use the ternary operator:

$user_id = ( $user_id === false) ? get_id_from_session() : $user_id;

Which is just a compact version of writing:

if( $user_id === false) {
    $user_id = get_id_from_session();
}

If you want to be real fancy and less-readable, you can omit the middle part (PHP > 5.3):

$user_id = ( $user_id) ?: get_id_from_session();

Now, if ( $user_id) evaluates to true, you'd get the value of $user_id in $user_id, otherwise you'd get the return value from the function.

answered Aug 6, 2012 at 17:29

nickb

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2 Comments

Zar Over a year ago

It sure shortens the code, but the readability becomes, in my opinion, much, much worse..

Karoly Horvath Over a year ago

@Zar: any other trick to achieve the same effect will ruin readability too.

drew010 · Accepted Answer · 2012-08-06 17:32:58Z

Default function arguments can only take constant values, not expressions that are evaluated at runtime so your second option won't work.

One idea I have would be something like this.

function foo($user_id = null) {
    $data = get_defaults('user_id' => $user_id);

    extract($data);

    echo $user_id; // should be the value returned from get_defaults()
}

The get_defaults() function might look like this:

function get_defaults(array $params) {
    if (isset($params['user_id']) && is_null($params['user_id'])) {
        $params['user_id'] = get_id_from_session();
    }

    if (isset($params['something_else']) && !isValidSomething($params['something_else'])) {
        $params['something_else'] = get_something_else();
    }

    return $params;
}

You would just pass an array to the get_defaults() function based on the arguments the specific called function expects. So another example would be:

function bar($baz, $user_id = null, $user_name = null, $return = null) {
    $data = get_defaults(array('return' => $return,
                               'user_name' => $user_name,
                               'user_id'   => $user_id));

    extract($data);

    // normal function code below
}

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