LUA Metatables

Metatables can be assigned to LUA tables to describe or change behaviour of operations on LUA tables. If table has metatable assigned, LUA engine looks for some special fields in metatables if some special events occurs. For example, if somebody wants to add two tables (system doesn't know how to add two tables because content is user dependent), LUA engine looks for special key __add in metatable to execute operator “+” and tries to execute funtion assigned to __add variable. In this case __add is a so called metamethod.

Possible LUA metamethods in metatable (see MetatableEvents):

• Arithmetic: __unm, __add, __sub, __mul, __div, __mod, __pow, __concat
• Comparison: __eq, __lt, __le
• Special: __index, __newindex, __mode, __call, __metatable, __tostring, __len, __gc

By default LUA table has no metatable assigned.

local NormalTable = {}
print(getmetatable(NormalTable))   
-- will print "nil"

Metatables are normal LUA tables

local NormalTable = {}
local MetaTable = {}
setmetatable (NormalTable, MetaTable)
print(getmetatable(NormalTable))   
-- will print: table: 0x20db880

If mettables are normal table, so it is possible to use the same table also as metatable to save some resources

setmetatable (NormalTable, NormalTable)

setmetatable(table, metatable) returns its first argument, so it is possible to simplify code below:

local NormalTable = {}
setmetatable (NormalTable, NormalTable)

to

local NormalTable = setmetatable ({}, {})

http://lua-users.org/wiki/MetatableEvents

http://lua-users.org/wiki/MetamethodsTutorial

• __index - is called when key in table is accessed. __index can be a function or another table - fallback table. Fallback table can have metatable which points to another fallback table and so on. It is possible to create very long fallback table chain.
  • __index = function (table, key), return value of fucntion is returned as result.
  • __index = table
  • to access table key without invoking __index methamethod use rawget(table,key)

• * __newindex is called when new value is assigned to key key
  • __newindex = function (table, key ,value)
  • to set new value without invoking methamethod, use rawset(table, key, value)

• __metatable when set, metatable is hidden. Value of __metatable is returned instead of original metatable.
• __call - if somebady calls table as function, this metamethod will be called
• __tostring - when tostring(table) is called
• __len

To satisfy programmers and increase code readability function can be declared using colon “:” syntax

Usual way of declaring function:

function class.method(self, a, b, c)
  -- ...
end

can be declared also as

function class:method(a, b, c)
  -- ...
end

Function calls are also possible using both methods:

class:method(1, 2, 3)
class.method(t, 1, 2, 3)