Using __gc to detect garbagecollection on non-userdata types

-- Keeping track of when any data is collected by lua's garbage collection
-- Only works for Lua version 5.1 as newproxy was removed in Lua 5.2

-- How this works:
-- Lua only calls the __gc metamethod for userdata values. This is because
-- C should be handling the destruction of userdata as Lua doesn't know how.
-- By using newproxy we can create our own userdata then append it to the
-- values metatable using debug.setmetatable (which allows one to set the
-- metatable of any type, not just tables). The idea is that when the object
-- is deleted, the metatable will be deleted as well, thus calling the __gc
-- metamethod.

-- In this example, I will show how one can do just this. Keep in mind if
-- you want to give the value a metatable you must keep the __userData metamethod.
-- Also note the __userData metamethod is NOT a real metamethod.

local table = {};

local function newUserdata(parent, key, value)
    -- Create a new userdata
    -- Takes a boolean argument to give it a metatable
    -- initially, but I won't add that just to make the
    -- example clearer.
    local userdata = newproxy();
    -- Set the metatable to an empty table.
    debug.setmetatable(userdata, {});
    -- Get the metatable that was just made.
    local udmeta = debug.getmetatable(userdata);
    -- Give it a __gc metatable which will be called
    -- when it is destoryed
    udmeta.__gc = function(ud)
        print("trashed:", ud, parent, key, value);
    end
    return userdata;
end

setmetatable(table,
    {
        -- When a new key is added to the table this will be called
        __newindex = function(tab, key, value)
            -- Set 'values' metatable and append a metamethod containing the userdata
            debug.setmetatable(value, {__userData = newUserdata(tab, key, value)});
            rawset(tab, key, value);
        end,
    }
);

table["example_key"] = "example_value";

print("test:", table["example_key"]);
-- prints: "test:   example_value"

print("collecting", collectgarbage("collect"));
-- After running the file twice the GC will collect the
-- previous table and print the following.
-- "trashed:    userdata: 0000  table: 0000 example_key example_value
-- "collecting  0"