Using 'Object' functions in Lua?

Question

I want to be able to have 'objects' with certain functions that refer to themselves (I have no idea what to call this) in Lua. I have seen code of what I'm trying to do but I have never understood what any of it actually means. I have tried looking over the Lua website but no luck.

Basic Code:

table = {}

function newTable(...)
  ...
  return setmetatable(table)
end

function table:funcName(...)
  ...
end

Can someone explain what is going on here and how I can use this please? Thanks for reading!

Answer 1

missingno already mentioned one resource that explains how this works. You can also check out lua wiki's OOP section for more explanation and examples.

To summary briefly your example, starting with how to use it. Note, I changed some of the names so it doesn't affect the standard modules that comes with lua. You create a new object by calling newObject. You can invoke that object's methods using : followed by the method name:

-- create two instances
object1 = newObject()
object2 = newObject()

-- call 'my_function' method for object1
object1:my_function()

You'll need to know a bit about metatables to understand how this machinery works behind the scenes. When you perform a call like:

object1:my_function()

That is just syntax sugar for:

object1.my_function(object1)

This can be broken down further into:

object1["my_function"](object1)

Now object1 is just an empty table returned by newObject -- it doesn't have a "my_function" key. Normally this would result in an error because you're trying to call a nil value. However, you can change this behavior using metatables. The basic idea is to setup the __index metamethod to point to a table that holds your class methods:

object_table = {}
object_table.__index = object_table
function newObject(...)
  return setmetatable({}, object_table)
end

The method lookup process will then look like this: object1 -> table. If object1 doesn't have a key, table is consulted next. If table has that key the associated value is returned. If table doesn't have it either then nil is returned since table doesn't have a metatable.

With this setup you can "override" a method for a particular object instance by just assigning the method name and function as key-value pair for that object.

object2.my_function = function (...)
  -- do something different
end