Swifty NSMapTable that can deal with weak referenced objects - protocol extension edition

ZMapTable.swift

// ZMapTable.swift
//
// Copyright (c) 2016 Kaz Yoshikawa. Released under MIT License.


protocol ZMapTableProtocol: CustomStringConvertible {
	associatedtype K: AnyObject
	associatedtype V: AnyObject
	
	var mapTable: NSMapTable { get }
	subscript(key: K) -> V? { get set }
}

extension ZMapTableProtocol {
	subscript(key: K) -> V? {
		get { return self.mapTable.objectForKey(key) as? V }
		set { self.mapTable.setObject(newValue, forKey: key) }
	}
	var description: String { return self.mapTable.description }
}


class ZStrongToWeakMapTable<T: AnyObject, U: AnyObject>: ZMapTableProtocol {
	typealias K = T
	typealias V = U
	let mapTable: NSMapTable = NSMapTable.strongToWeakObjectsMapTable()
}

class ZWeakToStrongMapTable<T: AnyObject, U: AnyObject>: ZMapTableProtocol {
	typealias K = T
	typealias V = U
	let mapTable: NSMapTable = NSMapTable.weakToStrongObjectsMapTable()
}

class ZWeakToWeakMapTable<T: AnyObject, U: AnyObject>: ZMapTableProtocol {
	typealias K = T
	typealias V = U
	let mapTable: NSMapTable = NSMapTable.weakToWeakObjectsMapTable()
}

class ZStrongToStrongMapTable<T: AnyObject, U: AnyObject>: ZMapTableProtocol {
	typealias K = T
	typealias V = U
	let mapTable: NSMapTable = NSMapTable.strongToStrongObjectsMapTable()
}