Undefined method error?-Collection of common programming errors

Joomla,I defined a method

function assignRef($key, &$val)
 {
  if (is_string($key) && substr($key, 0, 1) != '_')
  {
   $this->$key =& $val;
   return true;
  }

  return false;
 }

but when I cal that method I got an fatal error like:

WWDatingController::assignRef()

It is a Joomla base project. Please provide solution.