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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 as below, or enter http://melksh.am/nnnn for individual articles
U107 - Object Orientation - the Lua way
  [4753] Lua, Tcl, Python, C and C++ courses - at our Melksham HQ or on your site - forward from July 2017 - (2017-07-02)
  [4573] Classic style OO code - in Lua - (2015-11-05)
  [4572] Tables with values and code in Lua - looks like an object? - (2015-11-05)
  [4273] Dot or Colon separator between table name and member in Lua - what is the difference? - (2014-05-06)
  [4248] Metatables, Metamethods, classes and objects in Lua - (2014-03-18)
  [4117] Is Lua an Object Oriented language? - (2013-06-15)
  [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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