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- Subject: Re: Properly expanding lightuserdata in Lua 5.3.
- From: Andrew Gierth <andrew@...>
- Date: Sat, 09 Jun 2018 13:32:49 +0100
>>>>> "Sleepers" == Sleepers Tang <firstname.lastname@example.org> writes:
Sleepers> I am trying to write a Lua wrapper for my hobby game engine
Sleepers> in C++. Here is the thing.
Sleepers> How could I bind a table to lightuserdata?
Sleepers> I wanna expose a Lua API like this.
Sleepers> local sprite = require "engine.sprite" -- C module
Sleepers> local s = sprite.new() -- returns a C lightuserdata
You want that to be a full userdata, not a lightuserdata.
A full userdata with a metatable that defines __index (and __newindex,
__pairs and __len if you want those too) behaves like a table to lua
code; readonly if you only define __index, or writable if you define
You have a choice: __index and __newindex can be functions, which will
be called to fetch or store values; or you can store an actual lua table
Sleepers> but since Lua 5.3 have removed the setfenv,
Full userdata in 5.3 have a "uservalue" slot in place of the old
environment table slot; you can store a table there if you need one.
Such a table is not accessible from Lua (unless you provide functions to
access it) but it's a handy place for C functions to store Lua values
that need to be kept associated with the userdata.