This defines an array with strict bounds checking. It is also impossible to set any existing index to a nil value, so the array remains free of holes. The table functions will not work on such arrays because they are userdata, giving an extra layer of pro

safe_array.lua

local function out_of_range(i,n)
    if type(i) == 'number' then
        if i > 0 and i <= n then return true
        else error('out of range',3)
        end
    else
        error 'cannot index array by non-number type'
    end
end

function safe_array(n,val)
    local array = {}
    if n then
        val = val or 0
        for i = 1,n do array[i] = val end
    end
    local obj = newproxy(true)    
    local MT = getmetatable(obj)
    
    function MT.__index(o,i)
        out_of_range(i,#array)
        return array[i]
    end
    
    function MT.__newindex(o,i,v)
        -- note that assignment to array[#array+1] is permissible!
        out_of_range(i,#array+1)
        if v == nil then error('cannot assign nil',2) end
         array[i] = v
    end
    
    function MT.__len(t)
        return #array
    end
    return obj
end

obj = safe_array()

for i = 1,10 do
    obj[i] = 10*i
end

for i = 1,#obj do print(i,obj[i]) end

-- table.insert(obj,10) can't use table functions on userdata!
-- obj[16]=10 --out of range
--obj[5] = nil -- cannot assign nil