I'm trying to find a way to check if a Metatable is readonly or not
for example
local mt = metatable(game)
if mt == "readonly" do
print("Attempt to modify Metatables")
end
I hope there is a way to do this for Roblox, so I can prevent GUI tampering
You can use getmetatable() to see if the contents of a metatable are protected.
Example:
local mt = getmetatable(game)
if mt ~= nil and type(mt) ~= "table" then -- not perfect, as __metatable could be set to {}
print("This metatable is protected!")
end
Alternatively if you are looking to see if the table itself is read-only, you will need to check two behaviors
What happens when you attempt to add a value to the table
What happens when you attempt to change a value in the table.
Example of read-only table:
local protected_table = {'1', '2', '3'}
local table = setmetatable({}, {-- create a dumby table with a metatable
__index = function(_, k)
return protected_table[k]-- access protected table
end,
__newindex = function()
error('This value is read-only.')
end,
__pairs = function(_)
return function(_, k)
return next(protected_table, k)
end
end,
__metatable = false,
})
Examples of possible interactions:
table[4] = "4" -- read-only error will be generated by _newindex setting
table[1] = "0" -- read-only error will be generated by _newindex setting
first = table[1] -- will retrieve first value of protected_table
Related
I'm learning Lua and how to implement OOP. Trying out a test example of an object seems to return one of the argument of the object as 'null' despite being assigned one.
function Character(Name, Level, Class) --Constructor
return {GetName = T.GetName, GetLevel = T.GetLevel, GetClass = T.GetClass}
end
end
-- Snippets
Player = Character("Bob", 1, "Novice")
When I try printing Player.GetName() it returns null instead of Bob. Where have I gone wrong?
Here is the full code.
OOP in Lua takes a bit more than what you have done there, you will want to make use of metatables and upvalues.
-- How you could define your character structure.
local Character = {}
function Character.GetName(self)
return self.name
end
function Character.new(Name, Level, Class)
local _meta = {}
local _private = {}
_private.name = Name
_private.level = Level
_private.class = Class
_meta.__index = function(t, k) -- This allows access to _private
return rawget(_private, k) or rawget(Character, k)
end
_meta.__newindex = function(t, k, v) -- This prevents the value from being shaded
if rawget(_private, k) or rawget(Character, k) then
error("this field is protected")
else
rawset(t, k, v)
end
end
return setmetatable({}, _meta) --return an empty table with our meta methods implemented
end
This creates a local table _private when you create a new instance of a Character. That local table is an upvalue to the _meta.__index and it cannot be accessed outside the scope of the Character.new function. _private can be accessed when __index is called because it is an upvalue.
-- How to use the character structure
player = Character.new("Bob", 10, "Novice")
npc = Character.new("Alice", 11, "Novice")
print(player:GetName())
I use player:GetName(), but in all honesty you can just do player.name as well.
Resources for more on this topic:
http://tutorialspoint.com/lua/lua_metatables.htm
http://lua-users.org/wiki/ObjectOrientationTutorial
I make a function to parsing path name and file name from a computer directory using Lua Cheat Engine, next I want store the results in to a Lua table.
My function :
function addSongList()
load_dialog = createOpenDialog(self)
load_dialog.InitalDir = os.getenv('%USERPROFILE%')
load_dialog.Filter = 'MP3 files|*.mp3|*'
load_dialog.execute()
file = load_dialog.FileName
if file then
--- parsing path and filename
local pathN = file:match("(.*[\\/])")
local path, name = file:match('(.*\\)(.*)%.mp3')
--- test to open E:\MyMP3\mysong.mp3
print(pathN) --- result : E:\MyMP3\
print(name) --- result : mysong.mp3
end
end
local mp3Table = {}
table.insert(mp3Table,{pathN,name})
Is this correct way and correct syntax using table.insert(mp3Table,{pathN,name})
How to check if elements already added to the table by print out them?
How to clearing / empty the table ?
Thank you
1 - Inserting to the table:
table.insert(tb, value) inserts the value to de table tb. Using table.insert(mp3Table,{pathN,name}) you are dynamically creating a (sub)table and then appending to the main one.
2 - Printing the table.
As already pointed out you can just traverse the table using pairs or ipairs in order to get the elements.
I prefer ipairs in this case because the table is numerically indexed and order is guaranteed in accordance to table.insert.
The inner table must be indexed by numbers because you created it usign numeric indices.
for k, v in ipairs(mp3Table) do
print(v[1], v[2])
end
But you can also opt for a metatable which will also give you the possibility to generate a string representation for the table:
mp3Table_mt =
{
__tostring = function(self)
local ret = {}
for k, v in ipairs(self) do
table.insert(ret, v[1] .. "\t" .. v[2])
end
return table.concat(ret, "\n")
end
}
When initializing mp3Table you have to assign the metatable
local mp3Table = setmetatable({}, mp3Table_mt)
Then you can just tell Lua to print the table:
print(mp3Table)
3 - Empty/Delete the table:
Well there are two different things here. One is empty another is delete.
Lua uses garbage collection so actual deleting only occurs when there are no more references to a particular table. What you can do to tell Lua you no longer need a variable is assing nil to it. If there is no other reference to the value your variable was pointing to, the GC will clean it when it runs.
But you can empty the table without deleting it.
It may be tempting to say that mp3Table = {} "empties the table". But it does not.
What you are doing in this case is assigning a fresh new table to mp3Table variable. And if any other variable is still pointing to the old table it will no get collected and the inner values will remain untouched. If there's no such other variable, the table will be garbage collected just as if you assigned nil to mp3Table variable.
So to effectivelly empty a table you have to traverse it and set all its variables to nil.
function clearTable(tb)
for i, v in pairs(tb) do
tb[i] = nil
end
end
Specifically in the case asked, just assigning a new table to mp3Table may be enough because there are no more references to the same table. Assign nil afterwards is not necessary. What matters is if there are variables pointing to the same value. If you know what you are doing and the consequenses then no problem go ahead.
Putting it all together:
mp3Table_mt =
{
__tostring = function(self)
local ret = {}
for k, v in ipairs(self) do
table.insert(ret, v[1] .. "\t" .. v[2])
end
return table.concat(ret, "\n")
end
}
function addSongList(mp3Table)
local load_dialog = createOpenDialog(self)
load_dialog.InitalDir = os.getenv('%USERPROFILE%')
load_dialog.Filter = 'MP3 files|*.mp3|*'
load_dialog.execute()
file = load_dialog.FileName
if file then
--- parsing path and filename
local pathN = file:match("(.*[\\/])")
local path, name = file:match('(.*\\)(.*)%.mp3')
--- test to open E:\MyMP3\mysong.mp3
print(pathN) --- result : E:\MyMP3\
print(name) --- result : mysong.mp3
table.insert(mp3Table,{pathN,name})
end
return mp3Table
end
function clearTable(tb)
for i, v in pairs(tb) do
tb[i] = nil
end
end
local mp3Table = setmetatable({}, mp3Table_mt)
print(addSongList(mp3Table))
clearTable(mp3Table) -- I'm not assigning a new one. Just clearing the fields.
print(mp3Table) -- Must print nothing
1)yes
2)printing table in cycle:
for k,v in pairs(mp3Table) do
print( v.pathN, v.name)
end
3)empty table
mp3Table = {} -- clean
mp3Table = nil -- delete
I made a custom table using metatables that automatically tracks sizing when elements are added. It works very well and removes the need for the # operator or the getn function.
However it has a problem. If you call table.insert, the function apparently never calls __index or __newindex. Thus, my table cannot know when elements are removed this way. I assume the problem is the same with table.remove as well.
How can I either:
Capture the event of insert and use my own function to do so
Throw an error if insert is called on my table.
Thanks
function Table_new()
local public = { }
local tbl = { }
local size = 0
function public.size()
return size
end
return setmetatable(public, {
__newindex = function(t, k, v)
local previous_v = tbl[k]
rawset(tbl, k, v)
if previous_v ~= nil then
if v == nil then
size = size - 1
end
elseif v ~= nil then
size = size + 1
end
end,
__index = tbl
})
end
local t = Table_new()
t[5] = "hi"
t[17] = "hello"
t[2] = "yo"
t[17] = nil
print(t.size()) -- prints 2
local z = Table_new()
table.insert(z, "hey")
table.insert(z, "hello")
table.insert(z, "yo")
print(z.size()) -- prints 1 -- why?
If you print k,v in __newindex, you'll see that k is always 1. This is because table.insert asks for the size of table to find where to insert the value. By default, it's at the end. You should add a __len metamethod. But perhaps this defeats your purposes (which are obscure to me).
I am have a situation where two libraries L,M, are trying to set a metatable for _G (named mL, mM respectively). The only thing in the metatables are __index.
How can I chain these two metatables so that if the __index in one fails it calls the index in the other?
Have one metatable that stores both mL and mM, and if one returns nil, check the other:
local metatbl = {}
metatbl.tbls = {mL, mM};
function metatbl.__index(intbl, key)
for i, mtbl in ipairs(metatbl.tbls) do
local mmethod = mtbl.__index
if(type(mmethod) == "function") then
local ret = mmethod(table, key)
if ret then return ret end
else
if mmethod[key] then return mmethod[key] end
end
return nil
end
end
setmetatable(_G,metatbl)
Assuming there's a point where your code can fiddle with _G's metatable itself, after the libraries have mucked about, to fix what L and M did, you can just stick in your own metatable that does the combined search, e.g.:
combined_metatable = {
__index = function (t, k)
return mL.__index (t, k) or mM.__index (t, k)
end
}
setmetatable (_G, combined_metatable)
That has the advantage of not fiddling with mL or mM themselves.
If you don't have the opportunity to correct things after the fact, you could just modify the __index entries of the library metatables to do the combined search:
local original_mM_index = mM.__index
local original_mL_index = mL.__index
local function L_then_M_index (t, k)
return original_mL_index (t, k) or original_mM_index (t, k)
end
mL.__index = L_then_M_index
mM.__index = L_then_M_index
[Note that as both library metatables are modified, this will work whichever gets installed last ("winning" the competition).]
Use __metatable to give them a table that isn't actually the metatable or give the library a different setmetatable: that way they can't change your _G metatable.
getmetatable(getfenv()).__metatable = function ( o ) return { } end
OR
local orig_setmetatable = setmetatable
function setmetatable ( ob , mt )
if ob == getfenv() or ob == _G then
return ob
else
return orig_setmetatable(ob,mt)
end
end
(depending on how the library does things)
If you still want to track the things it does to the metatable; look through mt before the return ob (and if you actually wanted to chain __index lookups; add to a table):
local env_indexes = {}
setmetatable(_G,{__index=function(t,k) for i,m in ipairs(env_indexes) do local v=m[k]; if v then return v end end return nil end } )
local orig_setmetatable = setmetatable
function setmetatable ( ob , mt )
if ob == _G then
table.insert ( env_indexes , mt.__index )
return ob
else
return orig_setmetatable(ob,mt)
end
end
Otherwise this is very bad practice for libraries to be doing; tell the author not to!
I'm trying to write a metatable so that all indexes into the table are shifted up one position (i.e. t[i] should return t[i+1]). I need to do this because the table is defined using index 1 as the first element, but I have to interface with a program that uses index 0 as the first element. Since reading Programming in Lua, I think that I can accomplish what I want with a proxy table, but I can't seem to get it working. So far, I have this:
t = {"foo", "bar"}
local _t = t
t = {}
local mt = {
__index = function(t, i)
return _t[i+1]
end
}
setmetatable(t, mt)
However, this does not create the expected result. In fact, it doesn't return any values at all (every lookup is nil). Is there a better way to do this, or am I just missing something?
t = {"foo", "bar"}
local _t = t
t = {}
local mt = {
__index = function(t, i)
return _t[i+1]
end
}
setmetatable(t, mt)
print(t[0])
outputs "foo" for me when run here: http://www.lua.org/cgi-bin/demo