Custom Exception & get_instance()

The Problem is, that the whole CI-Exception class works like a standalone class without any CI core functionality like loading models, helpers and so on.

I get that. What I don't get is what you need from the core for your 404 page. Do you need the instance while the framework is in bootstrap phase - IOW, while executing CodeIgniter.php?

If your "need to call show_404() manually" happens after the framework is up and running then a custom 404 method in your extension of CI_Exception could be devised that will maybe do the trick.

PHP Code:

class MY_Exceptions extends CI_Exceptions
{

    /*
     * Accepts a third argument that would typically be '$this' when called from a running CI instance
     */
    public function show_custom_404($page = '', $log_error = TRUE, $sender = null)
    {
        if($sender)
        {
            //do stuff with the framework
            $sender->load->helper('url');
            $sender->load->view('my_swell_404_view');
        }
        else
        {
            parent::show_404($page, $log_error); //use the base class method
        }
    }
} 


You would need to create a helper to allow a simple function call in the way show_404() is used.
File: application/helpers/custom404_helper.php

PHP Code:

if ( ! function_exists('show_custom_404'))
{
 /**
 * A custom 404 Page handler that accepts a third argument
 *
 * @param string
 * @param bool
 * @param object An instance of CI ($this)
 * @return void
 */
 function show_custom_404($page = '', $log_error = TRUE, $sender = NULL)
 {
 $_error =& load_class('Exceptions', 'core');
 $_error->show_custom_404($page, $log_error, $sender);
 exit(4); // EXIT_UNKNOWN_FILE
 }
} 


So within some controller or other place in a fully instantiated framework (a place where $this is an instance of $CI)...

PHP Code:

show_custom_404($page, TRUE, $this); 


Does that help?