Custom Variable Types Lua

Question

I am looking for a library/function in lua that allows you to have custom variable types (even be detected as your custom type using the "type" method). I'm trying to make a json encoder/decoder that has the custom type "json." I want a solution that can be done in lua alone.

Answer 1

You cannot create new Lua types, but you can mimick their creation to a good extent using metatables and tables. For example:

local frobnicator_metatable = {}
frobnicator_metatable.__index = frobnicator_metatable

function frobnicator_metatable.ToString( self )
    return "Frobnicator object\n"
        .. "  field1 = " .. tostring( self.field1 ) .. "\n"
        .. "  field2 = " .. tostring( self.field2 )
end


local function NewFrobnicator( arg1, arg2 )
    local obj = { field1 = arg1, field2 = arg2 }
    return setmetatable( obj, frobnicator_metatable )
end

local original_type = type  -- saves `type` function
-- monkey patch type function
type = function( obj )
    local otype = original_type( obj )
    if  otype == "table" and getmetatable( obj ) == frobnicator_metatable then
        return "frobnicator"
    end
    return otype
end

local x = NewFrobnicator()
local y = NewFrobnicator( 1, "hello" )

print( x )
print( y )
print( "----" )
print( "The type of x is: " .. type(x) )
print( "The type of y is: " .. type(y) )
print( "----" )
print( x:ToString() )
print( y:ToString() )
print( "----" )
print( type( "hello!" ) )  -- just to see it works as usual
print( type( {} ) )  -- just to see it works as usual

Output:

table: 004649D0
table: 004649F8
----
The type of x is: frobnicator
The type of y is: frobnicator
----
Frobnicator object
  field1 = nil
  field2 = nil
Frobnicator object
  field1 = 1
  field2 = hello
----
string
table

Of course the example is simplistic and there are more things to say about object oriented programming in Lua. You may find the following references useful:

• Lua WIKI OOP index page.
• Lua WIKI page: Object Orientation Tutorial.
• Chapter on OOP of Programming in Lua. It is the first book edition, so it is focused on Lua 5.0, but the core material still applies.