Global variable named type is hiding the built-in type() function in Lua

Question

A C++ program I'm working on is using lua for configuration. It sets up several lua functions using a hardcoded script:

luaL_loadbuffer(pmLuaState, headerscript, strlen(headerscript), "header script");

It then loads the configuration lua file (that calls the previously mentioned functions):

luaL_loadfile(pmLuaState, filename)

Unfortunately, the Lua configuration file uses a global variable named type, so attempting to call the built-in type() function from my headerscript fails with a Lua error:

attempt to call global 'type' (a string value)

Due to the constraints of my scenario, I am unable to edit the offending config file. I'm wondering if there's a way to explicity state that I want to use the built-in type() function.

I'm new to Lua, so if there's a better way to load in these scripts that will avoid this global hiding issue, I'm open to it (but again, I cannot edit the config script).

Lua version: 5.1.4

Answer 1

Once the global variable has been modified, the value cannot be recovered- that is, you must prevent the script from setting _G.type in the first place, or store the value beforehand. The second is relatively trivial

local type = type -- problem solved

or you can use environments to solve this problem. This is a pretty common script to run.

local loadedmodules = {}
import = function(filename)
    if (loadedmodules[filename]) then
        return loadedmodules[filename]
    end
    local env = {}
    local func = loadfile(filename);
    local envmeta = {
        __index = _G -- changed thanks to comments
    }
    setmetatable(env, envmeta) -- Allow the imported script to read only our globals
    setfenv(func, env) -- set func's _G to env
    func() -- Load the script
    return loadedmodules[filename] = env -- Set the result and return it.
end

This would be the C++ equivalent of automatic header guards and macros being automatically scoped to the defining file.

Once you define this script from Lua, you can load it from C++ and call it relatively trivally. I don't recommend doing any extensive Lua logic from C++, the stack-based API is very hideous.