MiddleClass Equivalence

[allumbra]

Hello,

I recently discovered your MiddleClass lib and am wondering how you would implement object equivalence.

e.g.

local a = AnObject:new()
local b = a
if b==a then
print("objects are the same!")
end

[ninwa]

Since you cannot overload operators in Lua (I think?) you would need to implement a :compare member which parsed each tables structure and compared each individual element.

[bartbes]

You can overload operators, in this case you want to overload the __eq metamethod. (Problem is, if you don't know what it means, you'll need some reading up to do it.)

[Robin]

Since you cannot overload operators in Lua (I think?) you would need to implement a :compare member which parsed each tables structure and compared each individual element.

ORLY? You might want to take another look at chapter 13 of Programming in Lua, the revered PiL.

As for the OP, I'm not so sure. But I think you can use something like this

Code: Select all

--in a function, of course
for k, v in pairs(table1) do
  if type(table2[k]) ~= type(v) then
    return false
  elseif type(v) == 'table' then
    if not --[[thisFunction]](v, table2[k]) then
      return false
    end
   else
     if not v == table2[k] then
       return false
     end
   end
end
return true

This fails on recursive data structures, btw.

[kikito]

Hi there!

When did you download MiddleClass?

I noticed that problem with "pointer comparison" 10 days ago and made a small ammendment. The most recent version will run this code just fine.

Code: Select all

local a = AnObject:new()
local b = a
if b==a then
  print("objects are the same!")
end

You still can override the behaviour of the __eq operator by defining the __eq method in your class.

Code: Select all

function AnObject:initialize(name)
  super.initialize(self)
  self.name = name
end

-- redefine __eq
function AnObject:__eq(other)
  return self.name == other.name
end

-- create two different objects with the same name
local a = AnObject:new('peter')
local b = AnObject:new('peter')

if a == b
  print("the objects are equal")
end

In any case, I don't recommend this. Pointer comparison is too useful; I recommend defining a :equals method instead (a:equals(b)) and leaving __eq alone.

[Robin]

... except that in Lua you don't have pointers, so it's more like reference equality than pointer equality.

[kikito]

Indeed. I meant "reference comparison" not "pointer comparison". Wrote in a rush.

[allumbra]

Hey - that worked great. Apparently my lib was not up-to-date. Thanks for the prompt response and for the nice class lib

Sorry for the late reply - apparently I failed to submit my message previously

[ninwa]

I've been taken to school, thank you for the correction guys.

[Mud]

Indeed. I meant "reference comparison" not "pointer comparison". Wrote in a rush.

Well, the former is implemented in terms of the latter, so you weren't technical wrong.