Re: How to approach this?

[Andrew Starks <andrew.starks@...>]

On Wednesday, September 17, 2014, Jerome Vuarand <jerome.vuarand@gmail.com> wrote:
> 2014-09-17 10:10 GMT+01:00 Dirk Schippers <lua@frixx-it.com>:
> > The one point that was very important for me: lightuserdata can't have
> > metatables.
>
> In Lua all values can have metatables. But a crucial distinction is
> that some types have a per-value metatable, while other types have
> only one per-type (as in Lua's built-in types, not your own types).
> Full userdata belong to the first case. Each full userdata can have
> its own metatable, which they can share or not. Light userdata belong
> to the second case. There is only one metatable for all light
> userdata. Here is a quote from the manual:
>
>     Tables and full userdata have individual metatables (although
> multiple tables and userdata can share their metatables). Values of
> all other types share one single metatable per type; that is, there is
> one single metatable for all numbers, one for all strings, etc. By
> default, a value has no metatable, but the string library sets a
> metatable for the string type (see §6.4).
>
> Remember that light userdata belong to the second case, like numbers or strings.
>
> > That's strange because I created light userdata, I set a metatable to it and
> > my stack-debug showed that the light userdata had a metatable.
> > So although I had read that in the manual too, I thought the comment was
> > about something else.
>
> If you set the metatable of one light userdata, you're replacing the
> metatable of all other existing light userdata in the interpreter. If
> you check it just after setting it, it will be the value you expect.
> But as soon as you create another light userdata, and assign it a
> different metatable, the metatable for the first light userdata will
> be wrong.
>
> > The concept of the user values is new to me: I have searched the lua
> > programming guide and haven't found it, I only saw two functions in the
> > reference guide (lua_setuservalue and lua_getuservalue). Is there any more
> > info on this?
>
> The manual is indeed a bit lacking on that topic. Here is what the 5.1
> manual used to say (updated for 5.2):
>
>     [User value] associated with userdata have no meaning for Lua. It
> is only a convenience feature for programmers to associate a table to
> a userdata.
>
> As it says, it's just a convenience feature, which can be seen as
> redundant (you can do the same association with a weak table in the
> registry). It's typically used to store per-object information when
> the metatable stores per-class information. But you can do the
> opposite, or use them in any other way you see fit.

The most well-travelled path being:

Use a single metatable for all of the full userdata values in your *class*. 

Use a new uservalue (or upvalue) for each object in your class. Store per-object Lua data in here and configure your metamethods so that your user is writing to this table.  

Use light userdata to create a link between C values and Lua values, which can persist between calls to the Lua API, provided that your using the registry. 

-Andrew