Using Lua tables as objects

Archive - Originally posted on "The Horse's Mouth" - 2012-05-11 06:40:14 - Graham Ellis
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 ...

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Using Lua tables as objects