CODEWITHSUNDEEP
luaconstants
oiyio
oiyio

Reputation: 5925

is there any keyword like const or anything else which does the same job with it in lua?

Is there a const keyword in lua ? Or any other similar thing? Because i want to define my variables as const and prevent change of the value of the variables. Thanks in advance.

Upvotes: 54

Views: 45651

Answers (4)

Viliam Búr
Viliam Búr

Reputation: 2214

Update: November 2023

Following the release of Lua 5.4 in June 2020, support for constant variables has been added, refer to this answer by Ainar-G for more more information.

Original Answer

Lua does not support constants automatically, but you can add that functionality. For example by putting your constants in a table, and making the table read-only using metatable.

Here is how to do it: http://andrejs-cainikovs.blogspot.se/2009/05/lua-constants.html

The complication is that the names of your constants will not be merely "A" and "B", but something like "CONSTANTS.A" and "CONSTANTS.B". You can decide to put all your constants in one table, or to group them logically into multiple tables; for example "MATH.E" and "MATH.PI" for mathematical constants, etc.

Upvotes: 25

Ainar-G
Ainar-G

Reputation: 36199

I know this question is seven years old, but Lua 5.4 finally brings const to the developers! 

local a <const> = 42
a = 100500

Will produce an error: 

lua: tmp.lua:2: attempt to assign to const variable 'a'

Docs: https://www.lua.org/manual/5.4/manual.html#3.3.7.

Upvotes: 64

luastoned
luastoned

Reputation: 663

As already noted there is no const in Lua.

You can use this little workaround to 'protect' globally defined variables (compared to protected tables):

local protected = {}
function protect(key, value)
    if _G[key] then
        protected[key] = _G[key]
        _G[key] = nil
    else
        protected[key] = value
    end
end

local meta = {
    __index = protected,
    __newindex = function(tbl, key, value)
        if protected[key] then
            error("attempting to overwrite constant " .. tostring(key) .. " to " .. tostring(value), 2)
        end
        rawset(tbl, key, value)
    end
}

setmetatable(_G, meta)

-- sample usage
GLOBAL_A = 10
protect("GLOBAL_A")

GLOBAL_A = 5
print(GLOBAL_A)

Upvotes: 5

prapin
prapin

Reputation: 6858

There is no const keyword in Lua or similar construct.

The easiest solution is to write a big caution in a comment, telling that it is forbidden to write to this variable...

It is however technically possible to forbid writing (or reading) to a global variable by providing a metatable to the global environment _G (or _ENV in Lua 5.2).

Something like this:

local readonly_vars = { foo=1, bar=1, baz=1 }
setmetatable(_G, {__newindex=function(t, k, v)
  assert(not readonly_vars[k], 'read only variable!')
  rawset(t, k, v)
end})

Then if you try to assign something to foo, an error is thrown.

Upvotes: 2

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