Lua - C++ object metatable returns nil

Question

I have just started delving into Lua, and I learnt that C++ object properties could be accessible through metatables.

I am trying to access such an object's functions in a game's script: "GameLib". It is available in Lua as a global variable, but getmetatable() returns nil:

-- example usage elsewhere in the scripts:
local pPlayer = GameLib.GetLocalPlayer();

-- my tried code:
local mt = getmetatable(GameLib);
print("Metatable type:" .. type(mt)); -- "Metatable type: nil"

What could be the problem? Are there cases, when a C++ object has no metatable? If so, is there another way to access its properties?

Function `getmetatable` might be redefined to deliberately hide metatables from you (except metatables created by you). — Egor Skriptunoff, Apr 20 '21 at 21:43
Are you sure `GameLib` isn't just a regular old table? What game are you using Lua in? — Joseph Sible-Reinstate Monica, Apr 20 '21 at 22:12
@EgorSkriptunoff How common is that, compared to just using `__metatable` (which won't let you return a nil)? — Joseph Sible-Reinstate Monica, Apr 20 '21 at 22:15
@JosephSible-ReinstateMonica You are right, it was indeed a table, where function names are the keys and - mostly - the C++ pointers are the entries. I didn't know that functions can be called directly without indexing or anything like that. Thanks for the answers! — Zsolt DOKA, Apr 20 '21 at 23:17
@JosephSible-ReinstateMonica - Sometimes it is easier to redefine one function than insert additional field into 50 metatables. — Egor Skriptunoff, Apr 20 '21 at 23:49

score 2 · Accepted Answer · answered Apr 21 '21 at 08:57

From the Lua 5.4 Reference Manual:

2.4 Metatables and Metamethods:

Every value in Lua can have a metatable.

By default, a value has no metatable, but the string library sets a metatable for the string type

So there are cases where values, even userdata have no metatable. In fact that's default.

6.1 Basic Functions: getmetatable

If object does not have a metatable, returns nil. Otherwise, if the object's metatable has a __metatable field, returns the associated value. Otherwise, returns the metatable of the given object.

So the that leaves us with two options why getmetatable(GameLib) returns nil:

GameLib does not have a metatable
getmetatable is not Lua's getmetatable. It has been overwritten by a function that returns nil for at least some values. Trivial function getmetatable() end

Thanks for the elaborative answer! There was some confusion on my part as to how Lua handles structs that are usually objects in other business languages. C++ exported objects have the possibility to include their data in a metatable, which seemed exclusive; however the actual object representations can be of the type Lua table themselves, which I totally missed. — Zsolt DOKA, Apr 21 '21 at 21:36

Lua - C++ object metatable returns nil

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