default_controller is not working when enable_query_strings is enabled

[Derrekteng]

When you set $config['enable_query_strings'] = TRUE; in config/config.php
Then your default_controller will not work, even you've set these parameters.
$route['default_controller'] = 'home';
$route['404_override'] = '';

[Derrekteng]

And I researched about the source code, in the system/core/Router.php, the _set_rouing() function,
protected function _set_routing()
{
// Are query strings enabled in the config file? Normally CI doesn't utilize query strings
// since URI segments are more search-engine friendly, but they can optionally be used.
// If this feature is enabled, we will gather the directory/class/method a little differently
if ($this->enable_query_strings)
{
$_d = $this->config->item('directory_trigger');
$_d = isset($_GET[$_d]) ? trim($_GET[$_d], " \t\n\r\0\x0B/") : '';
if ($_d !== '')
{
$this->uri->filter_uri($_d);
$this->set_directory($_d);
}

        $_c = trim($this->config->item('controller_trigger'));
        if ( ! empty($_GET[$_c]))
        {
            $this->uri->filter_uri($_GET[$_c]);
            $this->set_class($_GET[$_c]);

            $_f = trim($this->config->item('function_trigger'));
            if ( ! empty($_GET[$_f]))
            {
                $this->uri->filter_uri($_GET[$_f]);
                $this->set_method($_GET[$_f]);
            }

            $this->uri->rsegments = array(
                1 => $this->class,
                2 => $this->method
            );
        }
        else
        {

/***************************

• NOTICE: here, if enabled enable_query_strings, and if there's no 'c' in the uri, it will call _set_default_controller directly, however, it has not parsed the config file yet, therefore, the $this->default_controller is null, which will cause

  protected function _set_default_controller()
  {
  if (empty($this->default_controller))
  {
  show_error('Unable to determine what should be displayed. A default route has not been specified in the routing file.');
  }
  ***************************/
  $this->_set_default_controller();
  }

      // Routing rules don't apply to query strings and we don't need to detect
      // directories, so we're done here
      return;
  }
  
  // Load the routes.php file.
  if (file_exists(APPPATH.'config/routes.php'))
  {
      include(APPPATH.'config/routes.php');
  }
  
  if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/routes.php'))
  {
      include(APPPATH.'config/'.ENVIRONMENT.'/routes.php');
  }
  
  // Validate & get reserved routes
  if (isset($route) && is_array($route))
  {
      isset($route['default_controller']) && $this->default_controller = $route['default_controller'];
      isset($route['translate_uri_dashes']) && $this->translate_uri_dashes = $route['translate_uri_dashes'];
      unset($route['default_controller'], $route['translate_uri_dashes']);
      $this->routes = $route;
  }
  
  // Is there anything to parse?
  if ($this->uri->uri_string !== '')
  {
      $this->_parse_routes();
  }
  else
  {
      $this->_set_default_controller();
  }
  

  }

Now I fix it like this, although it's not elegant:
protected function _set_routing()
{
// Are query strings enabled in the config file? Normally CI doesn't utilize query strings
// since URI segments are more search-engine friendly, but they can optionally be used.
// If this feature is enabled, we will gather the directory/class/method a little differently
if ($this->enable_query_strings)
{
$_d = $this->config->item('directory_trigger');
$_d = isset($_GET[$_d]) ? trim($_GET[$_d], " \t\n\r\0\x0B/") : '';
if ($_d !== '')
{
$this->uri->filter_uri($_d);
$this->set_directory($_d);
}

        $_c = trim($this->config->item('controller_trigger'));
        if ( ! empty($_GET[$_c]))
        {
            $this->uri->filter_uri($_GET[$_c]);
            $this->set_class($_GET[$_c]);

            $_f = trim($this->config->item('function_trigger'));
            if ( ! empty($_GET[$_f]))
            {
                $this->uri->filter_uri($_GET[$_f]);
                $this->set_method($_GET[$_f]);
            }

            $this->uri->rsegments = array(
                1 => $this->class,
                2 => $this->method
            );
        }
        else
        {
            /*
             * Derrekteng fix this, otherwise default_controller will not work if enable_query_strings is true
             */
            // Load the routes.php file.
            if (file_exists(APPPATH.'config/routes.php'))
            {
                include(APPPATH.'config/routes.php');
            }

            if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/routes.php'))
            {
                include(APPPATH.'config/'.ENVIRONMENT.'/routes.php');
            }

            // Validate & get reserved routes
            if (isset($route) && is_array($route))
            {
                isset($route['default_controller']) && $this->default_controller = $route['default_controller'];
                isset($route['translate_uri_dashes']) && $this->translate_uri_dashes = $route['translate_uri_dashes'];
                unset($route['default_controller'], $route['translate_uri_dashes']);
                $this->routes = $route;
            }
            $this->_set_default_controller();
        }

        // Routing rules don't apply to query strings and we don't need to detect
        // directories, so we're done here
        return;
    }

    // Load the routes.php file.
    if (file_exists(APPPATH.'config/routes.php'))
    {
        include(APPPATH.'config/routes.php');
    }

    if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/routes.php'))
    {
        include(APPPATH.'config/'.ENVIRONMENT.'/routes.php');
    }

    // Validate & get reserved routes
    if (isset($route) && is_array($route))
    {
        isset($route['default_controller']) && $this->default_controller = $route['default_controller'];
        isset($route['translate_uri_dashes']) && $this->translate_uri_dashes = $route['translate_uri_dashes'];
        unset($route['default_controller'], $route['translate_uri_dashes']);
        $this->routes = $route;
    }

    // Is there anything to parse?
    if ($this->uri->uri_string !== '')
    {
        $this->_parse_routes();
    }
    else
    {
        $this->_set_default_controller();
    }
}