Question

An example in the tutorial section : “Defining Metamethods for a C Type” looks as follows:

local ffi = require("ffi")
ffi.cdef[[
typedef struct { double x, y; } point_t;
]]

local point
local mt = {
  __add = function(a, b) return point(a.x+b.x, a.y+b.y) end,
  __len = function(a) return math.sqrt(a.x*a.x + a.y*a.y) end,
  __index = {
    area = function(a) return a.x*a.x + a.y*a.y end,
  },
}
point = ffi.metatype("point_t", mt)

local a = point(3, 4)

I’m a bit confused about where the “constructor” is, I assume by default point(3,4) is implicity routing 3 -> x and 5 -> y. What about when I want to hang some logic onto the constructor ? Put differently.. How do I specify a non-default constructor ?

I’m wrapping a bunch of c-libraries into object oriented lua code, and I do not care about portability to canonical lua. Specifically I need to hook in the three core functions of object oriented programming for object lifetime management, create , init, destroy. I know destroy will be the __gc method of my types’ metatable. So I need to know how to do create and init, and hopefully avoid a default initialization done by luajit.

edit

ffi.new and others have a bunch of rules governing the creation of types (documented in the luajit’s ffi semantics page). it’s in the semantics section. I’d still like to know what the cleanest way would be to hang custom creators and initializers (that come as part of a c library) into ffi object creation.

Answer 1

You’d need to wrap your point call to get what you desire:

local function newpoint ( vals )
    -- Do stuff with vals here?
    return point ( vals )
end
newpoint {x=5;y=4}

OR you could consider your point function as your create function; and just have an init method…

mt.__index.init = function ( p , x , y )
     p.x = x;
     p.y = y;
end

local mypoint = point()
mypoint:init ( 1 , 2 )

Note; all objects of the point type already have your metatable applied, so you dont need to attach methods or anything.

It does seem a bit pointless to me…. why do you want to seperate creation and initialization??