programming in lua, objects

Question

Sample code:

function Account:new (o)
  o = o or {}   -- create object if user does not provide one
  setmetatable(o, self)
  self.__index = self
  return o
end

taken from:

http://www.lua.org/pil/16.1.html

What is the purpose of the:

self.__index = self

line? And why is it executed every time an object is created?

Answer 1

They're used to re-direct table accesses (local y = table[key]) which are also used in method calls. In the above line, object o will have any attempts to access keys re-directed to the current object self, effortlessly inheriting all member functions. And possibly data variables too, depending on what exactly that __index is and how it works.

Answer 2

Lua is not an object-oriented language, but it has all of the facilities for writing object-oriented code. However, it is done in a prototyping fashion a la JavaScript. Instead of explicitly creating classes, a prototype object is created and then cloned to create new instances.

The __index metamethod is invoked to perform key lookups on read accesses to a table when the key is not already present in the table. So, self.__index = self essentially allows for inheritance of all methods and fields of the Account "class" by the new "class" that is created in the o = o or {} and setmetatable(o, self) lines.

See also:

• Metatables
• Metamethods Tutorial

Answer 3

As others have said, self (the Account table) is used as a metatable assigned to the objects created using new. Simplifying slightly (more info available at the links provided) when a field is not found in 'o', it goes to the 'Account' table because o's metatable says to go to Account (this is what __index does).

It does not, however, need to be executed every time an object is created. You could just as easily stick this somewhere:

Account.__index = Account

and it would work as well.

The somewhat longer story is that if an object o has a metatable, and that metatable has the __index field set, then a failed field lookup on o will use __index to find the field (__index can be a table or function). If o has the field set, you do not go to its metatable's __index function to get the information. Again, though, I encourage you to read more at the links provided above.