Lua metatable variable declaring

Question

I did not know what to name this question as I don't understand enough of what is going on.(Feel free to edit)

Consider the code below.

function object:new()
    o = o or {
        x = 0
    }
    setmetatable(o, self)
    self.__index = self
    self.y = 0

return o
end

table = object:new()

What are the differences between the variables (o.x and self.y) later on?

If I print_r the variable table, only the x is returned. However, both table.x and table.y can be accessed. This makes me realise that there is a difference between the two.

Could someone explain what the difference is and what reasons there are for putting variable in differant places?

Answer 1

What are the differences between the variables (o.x and self.y) later on?

You have two tables here, object and o.

Inside of object:new, self refers to the table object. So self.y is a field in the table object.

o is the new table you create in each call to object:new. o.x is a field in the table o.

That o table only has one entry: o["x"], so when you iterate over the entries in the table (as print_r does), that's all you're going to see.

So why does o.y give you a value? Because you set the table object as o's metatable and that metatable has it's __index field set, so when an index attempt fails on o, Lua will try again via o's metatable (if it has __index set).

A little code will probably make this clearer:

o = { x = 33 }

print(o.x, o.y) --> 33 nil

-- let's give o a metatable
mt = { y = 20 }
setmetatable(o, mt)

-- we've given o a metatable, but that metatable doesn't have an __index member set
print(o.y) --> nil

-- metatable `handlers` are normally functions that handle events
-- here we create a handler for indexing a table if the key doesn't exist in the table:
mt.__index = function(t,k)
  print("An attempt was made to index table", t, "with the key", k)
  return 5150
end

-- we've given o's metatable a handler for indexing which always returns 5150
print(o.x) --> 33
print(o.y) --> 5150
print(o.z) --> 5150
print(o.donut) --> 5150

-- note that Lua has a "rawget" function, which bypasses the metatable mechanics
print(rawget(o,'x')) --> 33, we get a value, because o actually contains "x"
print(rawget(o,'y')) --> nil, back to nil, because the metatable is being ignored

-- the __index metatable handler is special. Instead of providing a function
-- to handle missing key events, you can give it a table. If an index attempt fails,
-- it will try again in the __index table
mt.__index = mt -- we could have used a third table here, but mt has the y entry we want

-- we've changed the metatable handler, so now we'll get 777
print(o.y) --> 777

-- changes to the metatable are reflected in any object using it as metatable
mt.__index.y = 999
print(o.y) --> 999

-- now we give `o` it's OWN "y" entry, so the metatable handler will no longer be invoked
o.y = 2112

print(o.y) --> 2112
print(rawget(o, 'y')) --> o really owns this "y" entry
print(mt.__index.y) --> 999, the metatable was untouched by our write to o.y