LÖVE

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human = {age = 12}
student = {grade = 5}

student.__index = student
setmetatable(student,human)

function love.draw()
  love.graphics.print(""..student.age)
end

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do
	-- This object will represent the Human "class" and has all "fallback"
	-- values for table fields that human instances don't have.
	local Human = {age=12} -- This will be the __index metamethod.

	-- This will be the metatable that human instances will use.
	local HumanMetatable = {__index=Human}

	-- Here's an instance of our Human class. It's just a normal table so far.
	local someHuman = {}
	print("before setmetatable:", someHuman.age) -- nil, as we haven't set the metatable yet.

	setmetatable(someHuman, HumanMetatable)
	print("after setmetatable: ", someHuman.age) -- 12

	-- Because someHuman doesn't have an 'age' field (i.e. it's nil) Lua falls
	-- back to using the __index metamethod of the metatable (i.e.
	-- HumanMetatable.__index) to determine what value someHuman.age should
	-- return.

	-- Note that Lua simply returns HumanMetatable.__index.age here, i.e. Lua
	-- does a normal table lookup for an 'age' field in the table
	-- HumanMetatable.__index. (This will be important when we extend the
	-- Human class later.)
end

do
	-- As you see above, we're using two tables for the meta-functionality of
	-- our "human" objects - Human and HumanMetatable. This is not necessary -
	-- we can save a table by using Human as both the __index metamethod and
	-- the metatable itself. (This seems to cause some confusion.)

	local Human   = {age=12} -- Human is both the __index metamethod...
	Human.__index = Human    -- ...and the metatable!

	local someHuman = {}
	setmetatable(someHuman, Human) -- We can now use Human instead of HumanMetatable.
	print("human age:  ", someHuman.age) -- 12

	-- Now, let's extend the Human class with the subclass 'Student'.
	local Student   = {grade=5}
	Student.__index = Student -- Again with the same trick of reusing one table for both the metatable and the metamethod.

	-- We use setmetatable() the same way when we connect subclasses to parent
	-- classes as when we connect class instances to their class. We're just
	-- creating a table lookup "chain".
	setmetatable(Student, Human)

	local someStudent = {}
	setmetatable(someStudent, Student)
	print("student age:", someStudent.age) -- 12

	-- Here are all steps Lua takes to get the value 12:
	-- * Lua looks up someStudent.age - it's nil!
	-- * someStudent has a metatable, so...
	-- * Lua looks up getmetatable(someStudent).__index.age - it's nil!
	-- * getmetatable(someStudent).__index also has a metatable, so...
	-- * Lua looks up getmetatable(getmetatable(someStudent).__index).__index.age - it's 12!
	-- * Lua returns 12.
end

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Error: student.lua:8: attempt to concatenate field 'age' (a nil value)
stack traceback:
	[string "boot.lua"]:777: in function '__concat'
	student.lua:8: in function 'draw'
	[string "boot.lua"]:618: in function <[string "boot.lua"]:594>
	[C]: in function 'xpcall'

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student.age --nil so Lua going to look up the metatable of student.

setmetatable(student, human) --in this case the student has a metatable called human and so Lua go and look up the __index of human

human.__index = human --because the __index of human is point to the human table student.age will return a value  

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setmetatable(student,student) --in this case student is also the metatable and Lua going to look up the __index of student

student.__index = human --because the index of student is point to the human table, student.age return a value