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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Lua Metatables

In a recent article ([here]), I asked if Lua was an object oriented language - and I concluded that it supports the OO paradigm very well, even though it doesn't have keywords like "class", "private" or "extends". At the end of that article, I mentioned metatables - a subject I have carefully sidestepped so that I could show you the basics first. But Metatables are a vital piece of making Lua work "OO".

So - what is a metatable?

It's a table that you can add in to another ("regular" ?) table that adds members / characteristics to that original table - makes it do things differently.

For example, if you define a __tostring method in a metatable, then set that as the metatable for a regular table, you'll change how Lua prints out that table. I've put an example showing the mechanism [here].

How does a metatable work in an OO like way?

Let's have an example:
• set up a table that contains the common code for all objects of type "pet"
• set up a metatable that contains __string (and other stuff) which describes how a table that's set up to contain a pet performs its actions
• add a function (a constructor) which sets up another table for each time you call it - to contain all the data about a particular pety, and have that table reference the metatable just mentioned

You can then write common code to use pet objects ... the main application may be as simple as:
gypsy = pet.new("dog",28,3)
charles = pet.new("prince",95,60)
print (gypsy)
print (charles)

and it runs like this:
[trainee@holt lm10]$ lua metasmall
A pet of type dog aged 3 weighing 28
A pet of type prince aged 60 weighing 95
[trainee@holt lm10]$


The commented source showing you how to do that setup is [here].

How do I override operators?

The effect of adding a Gypsy to a Prince is a muddy mess - any you would like to do that using the "+" operator.

In Lua, you can override the standard operators for a table using functions such as __add in their metatable.

Add the code
ff = gypsy + charles
print (ff)

onto the end of the previous example, and run it:
[trainee@holt lm10]$ lua metapetite
A pet of type dog aged 3 weighing 28
A pet of type prince aged 60 weighing 95
Muddy mess
[trainee@holt lm10]$


The full code (showing the addition of the __add function) is [here]

Can I user methods to a series of tables?

The examples so far have overridden the behaviour of operators and standard functionallity - but you can also add your own. If you call a function that does not exist on a table, the __index function within its metatable is called up to resolve what code is to be run.

The first parameter to __index is the table on which the method is being called, and the second parameter is the name of the function needed. The __index method should return the function that's to be run (very often, that will be a function in the metatable itself, or in the pet table), or a nil if the requested function does not exist.

There's an example [here] - adding a getliftable method which returns a true or false value. The calling code looks like this:

gypsy = pet.new("dog",28,3)
charles = pet.new("prince",95,60)
family = {gypsy, charles}
 
for _,v in ipairs(family) do
  print (v)
  if v:getliftable() then
    print ("Can lift ")
  else
    print ("is very heavy")
  end
end


And when you run it, you get

[trainee@holt lm10]$ lua metamid
A pet of type dog aged 3 weighing 28
Can lift
A pet of type prince aged 60 weighing 95
is very heavy
[trainee@holt lm10]$


How does it decide whether to return true or false? Ah - that's the beauty of encapsulation which hides code within a set of fuctions that the higher level user calls.

Does getliftable always runs the same code? Not necessarily - if you had not only pets but also some other type of tables, Lua will select the appropriate code to run depending on the data type. That's polymorphism.

So - although you don't have all the usual keywords associated with an OO language, you do have the key feature / benefits - encapsulation, polymorphism, separation of detailed code for higher level user (I need to add some "local" declarations for better partitioning!) ... so I'm going to say - with confidence - that you can write code using OO techniques in Lua.

(written 2010-04-02)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
U107 - Object Orientation - the Lua way
  [1692] Towards Object Oriented Programming in Lua - (2008-06-30)
  [1699] If you are learning Lua, here are some more examples - (2008-07-06)
  [1743] First class functions in Lua lead to powerful OO facilities - (2008-08-07)
  [1819] Calling base class constructors - (2008-10-03)
  [2318] For Lua Programmers AND for Town Planners - (2009-08-02)
  [2359] A fresh example - objects the Lua way - (2009-08-13)
  [2455] Lua examples - coroutines, error handling, objects, etc - (2009-10-15)
  [2701] Is Lua an Object Oriented language? - (2010-04-01)
  [2710] __index and __newindex in Lua - metatable methods - (2010-04-05)
  [3142] Private and Public - and things between - (2011-01-22)
  [3396] Tables as Objects in Lua - a gentle introduction to data driven programming - (2011-08-17)
  [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17)
  [3683] Weak references in Lua - what are they, and why use them? - (2012-04-04)
  [3694] Special __ methods you can use in Lua metatables - (2012-04-12)
  [3727] Using Lua tables as objects - (2012-05-11)
  [3730] What is a metatable? How do I set one up? How do I use them? Lua - (2012-05-12)
  [4117] Is Lua an Object Oriented language? - (2013-06-15)
  [4248] Metatables, Metamethods, classes and objects in Lua - (2014-03-18)
  [4273] Dot or Colon separator between table name and member in Lua - what is the difference? - (2014-05-06)
  [4572] Tables with values and code in Lua - looks like an object? - (2015-11-05)
  [4573] Classic style OO code - in Lua - (2015-11-05)
  [4753] Lua, Tcl, Python, C and C++ courses - at our Melksham HQ or on your site - forward from July 2017 - (2017-07-02)

U105 - Lua - Tables and the table library.
  [1697] Sorting in lua - specifying your own sort routine - (2008-07-05)
  [1742] Lua - Table elements v table as a whole - (2008-08-07)
  [2346] The indexed and hashed parts of a Lua table - (2009-08-10)
  [2499] ourdog is Greyhound, Staffie and Ginger Cake - (2009-11-09)
  [2699] Lua tables - they are everything - (2010-03-30)
  [3725] Lua Tables - (2012-05-10)
  [4571] Lua - using modules to add your own utilities - (2015-11-04)


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The same very simple program in many different programming languages
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