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Special __ methods you can use in Lua metatables

In Lua, you can associate an additional table (known as a metatable) with each table of data and code you have. By doing so, repeated values and functions that apply across as whole series ("type", "class") of tables only have to be defined once. A very neat way for Lua to provide the OO facilities within a lightweight language.

Metatables are provided with a number of hooks, starting with __ (double underscore) which provide special functionallity / syntactic icing within the language; I believe that the following is a comprehensive list of them for Lua 5.2

__gc - finaliser (prior to garbage collection; the destructor)
__mode - setting keys or values to be weak

__add - addition
__sub - subtraction
__mul - multiplication
__div - division
__mod - modulo
__pow - raise to power

__unm - Unary Minus
__len - the # operator and the len function

__concat - concatenation (the .. operator)

__eq - equal
__lt - less than
__le - less than or equal

__index - Accessing a table element
__newindex - Creating a table element
__call - calling the code in a table element

__tostring - conversion to string (tostring and print)
__ipairs - the ipairs function
__pairs - the pairs function

__metatable - returns the metatable

See The Lua 5.2 manual for details of each.

(written 2012-04-12)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
U105 - Lua - Tables and the table library.
  [4571] Lua - using modules to add your own utilities - (2015-11-04)
  [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)
  [3725] Lua Tables - (2012-05-10)
  [2703] Lua Metatables - (2010-04-02)
  [2699] Lua tables - they are everything - (2010-03-30)
  [2499] ourdog is Greyhound, Staffie and Ginger Cake - (2009-11-09)
  [2346] The indexed and hashed parts of a Lua table - (2009-08-10)
  [1742] Lua - Table elements v table as a whole - (2008-08-07)
  [1697] Sorting in lua - specifying your own sort routine - (2008-07-05)

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)
  [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)
  [3727] Using Lua tables as objects - (2012-05-11)
  [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)
  [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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