Why does do_action pass a blank string as the first parameter if no $arg is set?

Question

I'm having a hard time understanding how to set up a function with a default parameter and then call it with the do_action in WordPress. I'm not sure if this is a WordPress issue or just me not understanding the do_action method. What happens is I define a function like this:

function contact_info( $show_logo=true ) {

    var_dump( $show_logo );    
}

Then I add the hook like this:

add_action( 'show_contact_info', 'contact_info' );

Finally when I call the do_action like this:

do_action('show_contact_info');

I would expect a the var_dump to be bool(true), but what I am getting is: string(0) ""

So my question is WHY?

It doesn't seem to be accepting the default as set in my function, or more specifically, the do_action call ALWAYS seems to pass blank as the first parameter if one is not defined in the call. Is this standard for Wordpress? Why would it not pass NULL?

Answer 1

When we fire up:

do_action( 'foo' );

we are actually calling:

do_action( 'foo', '' );

That's because how it's defined; With the empty string as the default input argument:

function do_action($tag, $arg = '') {
    // ...
}

So the action's callback get's the empty string as it's first input argument.

This has been like this since at least WordPress version 1.5.

Since WordPress 4.6, do_action() is a wrapper of WP_Hook::do_action();

What if ...

It looks like if it would be defined without the argument part as:

function do_action($tag) {
    // ...
}

and with corresponding modifications to this part:

$args = array();
if ( is_array($arg) && 1 == count($arg) && isset($arg[0]) && is_object($arg[0]) ) // array(&$this)
    $args[] =& $arg[0];
else
    $args[] = $arg;

then we might use the default value, of the first input argument, in our callback.