Using Lua tables as objects

Since tables in Lua can hold data members of different types, they can be used as the base structure for an Object Oriented design approach, where each table represents an object with numbers, strings, other tables ("objects") and callable blocks ("functions" or methods) as members. And if you take it one step further and put all the common code relating to a type of data into another table that's named after the data type, you can in effect design a class.

There's a new example of this [here] on our web site, from yesterday's Lua Course.

I created a series of "archive access objects" from our web server log file, where each object is a blog archive request to the server. The code to create each object (and add it to a table of such objects) is:
pages[#pages+1] = archive.new(num,nam)
and I then looped through all members of the table or archive objects, printing out their details:

  for _,access in pairs(pages) do

    print (access:getnum(), access:getnam())

  end

As ever with object orientation, the clever stuff is hidden within the class - in this case the archive table which I defined to hold a piece of code called new. new sets up a new table each time it's called, into which the data, and also a reference to functions getnum and getnam, are stored.

  archive.new = function (index, title)

    local current = {}

    current.index = index 

    current.title = title

    current.getnum = archive.getnum

    current.getnam = archive.getnam

    return current

  end

(the originals of getnam and getnum are stored in archive purely for convenience)

Rather than copy all the methods across into each object, Lua includes a facility called metatables that I'll be going on to look at during today's course ...
(written 2012-05-11)

Associated topics are indexed under

U107 - Object Orientation - the Lua way
  [3730] What is a metatable? How do I set one up? How do I use them? Lua - (2012-05-12)
  [3694] Special __ methods you can use in Lua metatables - (2012-04-12)
  [3683] Weak references in Lua - what are they, and why use them? - (2012-04-04)
  [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17)
  [3396] Tables as Objects in Lua - a gentle introduction to data driven programming - (2011-08-17)
  [3142] Private and Public - and things between - (2011-01-22)
  [2710] __index and __newindex in Lua - metatable methods - (2010-04-05)
  [2703] Lua Metatables - (2010-04-02)
  [2701] Is Lua an Object Oriented language? - (2010-04-01)
  [2455] Lua examples - coroutines, error handling, objects, etc - (2009-10-15)
  [2359] A fresh example - objects the Lua way - (2009-08-13)
  [2318] For Lua Programmers AND for Town Planners - (2009-08-02)
  [1819] Calling base class constructors - (2008-10-03)
  [1743] First class functions in Lua lead to powerful OO facilities - (2008-08-07)
  [1699] If you are learning Lua, here are some more examples - (2008-07-06)
  [1692] Towards Object Oriented Programming in Lua - (2008-06-30)

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Now Open - A Museum for Melksham. Come in and see us.
Then and now pictures of Melksham - on show through the summer
The future needs for rail services to Melksham - change needed; current service an insult
Using Lua tables as objects
Press Release - Museum to explore the story of Melksham
Lua Tables
Learning to Program in Lua - public / open training course / class
Bank Holiday Monday, so it was pouring with rain.
Walking by the wiver

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