Lua references between objects

Question

I implemented Simple Lua class in C. Usage of class:

require("test")
function foo()
    local t1 = test()
    local t2 = test()
    t1:attach(t2)
    return t1
end
o = foo()
-- some code
o = nil

attach function:

int class_attach(lua_State *L)
{
    module_data_t *mod = luaL_checkudata(L, 1, "test");
    luaL_checktype(L, 2, LUA_TUSERDATA);
    module_data_t *child = lua_touserdata(L, 2);
    printf("%p->%p\n", (void *)mod, (void *)child);
    return 0;
}

After return from function t2 object is cleaned by gc. Is it possible to prevent that. Set reference between t1 and t2 objects? (calling __gc metamethod (of t2 object) only after parent module (t1) is cleaned).

Simple way is use table:

function foo()
    ret = {}
    ret[1] = test()
    ret[2] = test()
    ret[1]:attach(ret[2])
    return ret
end

but that is not fun way. Thanks!

Answer 1

I've had the same issue in the past. The simple reason for this is that Lua doesn't know about the connection established in C code (there's none right now, but I guess it's going to be done somewhere).

You don't have to do the table approach, just link the two objects/tables/whatever on the Lua side as well:

function foo() 
    local t1 = test() 
    local t2 = test() 
    t1:attach(t2)
    t1._dummy_link = t2
    return t1 
end 

Just keep in mind that's only good for 1:1 relationships. For more complex stuff you'll still have to use some kind of table or a similar approach. The probably cleanest way to do this would be doing the linking on the Lua side and just adding a callback to C in case there's C code to be run as well.

Answer 2

You can set it in the Lua registry. This is effectively a global table which can only be accessed in C code. You can set registry[t1] = t2; . As long as you unset this appropriately in t1 's __gc . This can also scale to 1:n mappings, as you could do, for example, registry[t1].insert(t2) for multiple "children".