Plugin Class Demo

plugin-class-demo.php

<?php # -*- coding: utf-8 -*-
/**
 * Plugin Name: Plugin Class Demo
 * Description: How I am using the base class in plugins.
 * Plugin URI:
 * Version:     2012.09.29
 * Author:      Fuxia Scholz
 * License:     GPL
 * Text Domain: plugin_unique_name
 * Domain Path: /languages
 */

/*
 * I prefer the empty-constructor-approach showed here.
 *
 * Advantages:
 * 	- Unit tests can create new instances without activating any hooks
 *    automatically. No Singleton.
 *
 *  - No global variable needed.
 *
 *  - Whoever wants to work with the plugin instance can just call
 *    T5_Plugin_Class_Demo::get_instance().
 *
 *  - Easy to deactivate.
 *
 *  - Still real OOP: no working methods are static.
 *
 * Disadvantage:
 * 	- Maybe harder to read?
 *
 */

add_action(
	'plugins_loaded',
	array ( T5_Plugin_Class_Demo::get_instance(), 'plugin_setup' )
);

class T5_Plugin_Class_Demo
{
	/**
	 * Plugin instance.
	 *
	 * @see get_instance()
	 * @type object
	 */
	protected static $instance = NULL;

	/**
	 * URL to this plugin's directory.
	 *
	 * @type string
	 */
	public $plugin_url = '';

	/**
	 * Path to this plugin's directory.
	 *
	 * @type string
	 */
	public $plugin_path = '';

	/**
	 * Access this plugin’s working instance
	 *
	 * @wp-hook plugins_loaded
	 * @since   2012.09.13
	 * @return  object of this class
	 */
	public static function get_instance()
	{
		NULL === self::$instance and self::$instance = new self;

		return self::$instance;
	}

	/**
	 * Used for regular plugin work.
	 *
	 * @wp-hook plugins_loaded
	 * @since   2012.09.10
	 * @return  void
	 */
	public function plugin_setup()
	{

		$this->plugin_url    = plugins_url( '/', __FILE__ );
		$this->plugin_path   = plugin_dir_path( __FILE__ );
		$this->load_language( 'plugin_unique_name' );

		// more stuff: register actions and filters
	}

	/**
	 * Constructor. Intentionally left empty and public.
	 *
	 * @see plugin_setup()
	 * @since 2012.09.12
	 */
	public function __construct() {}

	/**
	 * Loads translation file.
	 *
	 * Accessible to other classes to load different language files (admin and
	 * front-end for example).
	 *
	 * @wp-hook init
	 * @param   string $domain
	 * @since   2012.09.11
	 * @return  void
	 */
	public function load_language( $domain )
	{
		load_plugin_textdomain(
			$domain,
			FALSE,
			$this->plugin_path . '/languages'
		);
	}
}

Quick question about this, sorry if it seems obvious or is really basic – but is there a reason to even have an empty __construct() for the Constructor, if plugin_setup() here is setting things up and doing all the plugin actions?

I'm just starting to learn about OOP and see information about __construct() methods in places like this StackOverlow answer that say it should be used for setting class properties – or is this true only if the properties as passed as arguments to the class, like in that answer I linked to above?

To clarify:

1. Is __construct() needed for defining a class's properties?
2. If it's wholly not needed, then why leave an empty version of it in the class at all, instead of just leaving property definitions, hooks, actions, etc. in a custom init method like this demo's plugin_setup() method?