How can I implement a read-only table in lua? - lua

Question

I wrote an example.
function readOnly(t)
local newTable = {}
local metaTable = {}
metaTable.__index = t
metaTable.__newindex = function(tbl, key, value) error("Data cannot be changed!") end
setmetatable(newTable, metaTable)
return newTable
end
local tbl = {
sex = {
male = 1,
female = 1,
},
identity = {
police = 1,
student = 2,
doctor = {
physician = 1,
oculist = 2,
}
}
}
local hold = readOnly(tbl)
print(hold.sex)
hold.sex = 2 --error
It means that I can give access to the field of the table "tbl" but at the same time, I cannot change the value related to the field.
Now, the problem is that I wanna let all the nested tables own this read-only
property.How can I improve the "readOnly" method?

Answer 1

You just have to apply your readOnly function recursively to the inner table fields as well. You can do so on-access in the __index metamethod. You should also cache the readonly proxy tables that you create, otherwise any read access to inner tables (e.g. hold.sex) will create a new proxy table.
-- remember mappings from original table to proxy table
local proxies = setmetatable( {}, { __mode = "k" } )
function readOnly( t )
if type( t ) == "table" then
-- check whether we already have a readonly proxy for this table
local p = proxies[ t ]
if not p then
-- create new proxy table for t
p = setmetatable( {}, {
__index = function( _, k )
-- apply `readonly` recursively to field `t[k]`
return readOnly( t[ k ] )
end,
__newindex = function()
error( "table is readonly", 2 )
end,
} )
proxies[ t ] = p
end
return p
else
-- non-tables are returned as is
return t
end
end