Inheritance, Encapsulation, and Classes in LUA

gistfile1.lua

--[[
  Multi level inheritance implemented using only
  tables and meta tables in lua

  The implementation follows the very simple scheme:

  class_inheritancelevel_n = {
    new = function(self, inh)
      new = {}
      for k, v in pairs(self) do
        new[k] = v
      end
      -- default values for fields
      new.vals = {}
      -- inheritance
      new.meta = {__index = new }
      if inh then setmetatable(inh, new.meta)
      
      if inh then return inh else return new end
    end

    -- [some fields and some methods]

    -- this is mandatory for all derivations
    meta = nil
  }

  ---------------------------------------
  Alterations with respect to oop_02.lua:
  privated fields!!!
]]

matrix = {
	new = function(self)
		-- private fields
		meta = {}
		vals = {}
		return function(inh)
			new = {}
			for k, v in pairs(self) do
				new[k] = v
			end
			-- defaults for the fields
			meta = { __index = new }

			-- operations		
			new.add = function(self, this_matrix)
				local svals = #vals
				for k=1, #this_matrix do
					vals[k + svals] = this_matrix[k]
				end
			end

			new.get_length = function(self)
				return #vals
			end

			new.get_vals = function(self)
				local copy = {}
				for k, v in pairs(vals) do
					copy[k] = v
				end
				return copy
			end

			new.tostring = function(self)
				local res = ""
				for k, v in pairs(vals) do
					res = res .. v .." "
				end
				return res
			end

			-- inheritance
			if inh then
				setmetatable(inh, meta)
			end
			-- new table instance
			if inh then
				return inh
			else
				return new
			end
		end
	end

	-- public fields
	-- ....
}

matrix_i1 = {
	new = function(self)
		-- private fields
		meta = {}
		counter = 0
		return function(inh)
			new = {}
			for k, v in pairs(self) do
				new[k] = v
			end
			new.i2_counter = 43
			
			-- methods
			new.get_vals_as_string = function(self)
				local res = ""
				for k, v in pairs(self.get_vals()) do
					res = res .. v
				end
				return res
			end

			new.get_counter = function(self)
				return counter
			end

			new.set_counter = function(self, val)
				counter = val
			end

			-- inheritance
			meta = { __index = new }
			if inh then
				setmetatable(inh, meta)
			end

			if inh then
				return inh
			else
				return new
			end
		end
	end,

	-- public fields
	i2_counter = nil
}

matrix_i2 = {
	new = function(self)
		-- private fields and methods
		meta = {}
		return function(inh)
			new = {}
			for k, v in pairs(self) do
				new[k] = v
			end
			-- inheritance
			meta = { __index = new }
			if inh then
				setmetatable(inh, meta)
			end

			if inh then
				return inh
			else
				return new
			end
		end
	end,

	-- public fields and methods without access to private data structs
	moo = function(self) print(self) end
}

m = matrix:new()()
m:add{1,2,3}
m:add{"Foo", "bar"}
print("m: (first) " .. m:tostring())

m = matrix:new()(matrix_i1:new()())
m:add{1,2,3}
m:add{"Foo", "bar"}
-- set_counter is private for matrix_i1
m:set_counter(34543234)
print("mi1: counter:" .. m:get_counter())
print("mi1: (second) " .. m:tostring())
print("mi1: (third) " .. m:get_vals_as_string())

-- determins the following inheritance relationships
-- matrix <- matrix_i1 <- matrix_i2
m = matrix:new()(matrix_i1:new()(matrix_i2:new()()))
m:add{1,2,3}
m:add{"Foo", "bar"}
print("mi2: (fourth)" .. m:tostring())
m:moo()