Question: Is there a C++ way to query the metatable of a sol::protected_function?

[joergbrech]

Thanks a lot for this amazing library!

With sol2, you can query the metatable of tables and table proxys using my_table[sol::metatable_key]. Is there any way to query the metatable of a sol::protected_function directly using the sol API? It doesn't implement operator[] and I found no way to convert it to a table or proxy type. A generic function like sol::getmetatable would be cool.

My use case is that I want to attach metadata to my C++ functions like so:

template <typename Table, typename Key, typename T>
void add_metadata(sol::state_view& lua, Table&& t, Key&& key, T&& value)
{
    if (!t[sol::metatable_key].valid()) {
        sol::table func_meta = lua.create_table();
        t[sol::metatable_key] = func_meta;
    }
    t[sol::metatable_key][std::forward<Key>(key)] = std::forward<T>(value);
}

int main()
{
    sol::state lua;
    lua.open_libraries(sol::lib::base);

    auto g = lua.globals();
    lua.set_function("bar", [](){});
    add_metadata(lua, g["bar"], "foo", "baz");
    
    foobar(lua, g["bar"]); // see below

    return 0;
}

Now I want to query this metadata in a context (function body of foobar), where I don't have g["bar"], but rather a sol::protected_function. I managed to retrieve the metatable with sol::state::script and one line of LUA code, but it would be nice to have something in the C++ API (which might boost performance as well?):

template <typename Table, typename Key>
auto get_metadata(sol::state_view& lua, Table&& t, Key&& key)
{
    // This works, but I was hoping for a cleaner solution
    lua["_tmp"] = t;
    lua.script(R"(
        _tmp = getmetatable(_tmp)
    )");
    sol::object ret = lua["_tmp"][std::forward<Key>(key)];
    return ret;
}

void foobar(sol::state_view& lua, sol::protected_function const& f)
{
    // in this context, I don't have access to f as a  proxy or table
    std::cout << get_metadata(lua, f, "foo").as<std::string>() << "\n";
}

Run this on godbolt

[shohnwal]

Does that actually work, can functions have metatables? I just checked source for Lua 5.1, 5.4 and LuaJit and I don't see any fields where functions could store a metatable. Tables on the other hand have clearly commented metatable fields.

When I try to do that with

function ClassA:MyFunc()
   setmetatable(self.MyFunc, { data = "meow!" })
end

I get an error bad argument #1 to 'setmetatable' (table expected, got function)

or does this only work with specific Lua versions? (I'm using LuaJit 2.1)
Or does the TableProxy + C++-template work in some wacky magic way when it comes to attaching/getting a metatable to/from functions?

That being said, being able to attach metadata to functions would be an awesome Lua feature.