Exploring accessing metamethods with FFI objects

FFI_metamethod_play.lua

local ffi = require 'ffi'
local C = ffi.C

-- in reality this would be some hashing library,
-- but for this example they are dumb functions
local hasher = function(x) return tonumber(x) end
local H = {
    hash = hasher,
    hash_combine = function(seed, v) return seed + hasher(v) end,
}

local Foo_t = ffi.metatype( ffi.typeof('struct { int a, b; }'), {
    __hash = function( self )
        return H.hash_combine( H.hash(self.a), H.hash(self.b) )
    end
})

local Bar_t = ffi.metatype( ffi.typeof('struct { int a, b; }'), {
    __index = {
        __hash = function( self )
            return H.hash_combine( H.hash(self.a), H.hash(self.b) )
        end
    }
})

local Baz_t = ffi.typeof('struct { int a, b; }')

-- getmetatable returns the string 'ffi' so we can't do anything with it
print( getmetatable(Foo_t()) )  

-- prints 'function'
print( type(Bar_t().__hash) )

-- without a metatable with __index, accessing __hash throws an error
print( pcall(function() return type(Baz_t().__hash) end ) )