Re: How to export associative arrays to an embedded lua script?

[Philipp Janda <siffiejoe@...>]

Am 10.10.2015 um 08:22 schröbte linuxfan@tin.it:
> Hi list,

Hi!

> [...]
>
> This creates a Lua table named "env". If a script tries to read a
> value from table env:
>
> print (env.screenwidth)
>
> lua nicely calls my routine xl_getxtable()
> with two arguments - the table and the key.
> My problem is that I don't know how to get the table name lua refers
> to; while the key is passed as a string, the table is passed as a lua
> internal variable which means nothing to me. I tried to search around
> in examples, docs, and so on, but with no success. I only discovered
> the Registry where, may be, I can store the table as key and its name
> as value, so I can later fetch the name of the table.

You could just store a name in the `env` table you create and look it up 
in your `__index`/`__newindex` functions. To prevent collisions with the 
normal keys you should use a non-string key (like a boolean or a number) 
for this though. Your registry idea is better, because it naturally 
prevents such collisions, and it is safer because the registry is 
inaccessible from non-debug Lua code.

> Probably I will end up using different xl_getxtable() functions, each
> one with its hard-coded table name inside; it would be faster and the
> tables to export are not so many. But while am at it, I want to
> understand.

This should work as well. You can even share the same code for all your 
different `__index` functions by storing the name in an upvalue. Instead 
of `lua_pushcfunction(L, xl_getxtable)` just use

    lua_pushstring(L, name);
    lua_pushcclosure( L, xl_getxtable, 1);

and access the name inside `xl_getxtable` using `lua_upvalueindex(1)`. 
Downside is that you need a different metatable for each pascal table 
you want to export. Also, I'm not sure that having a name as a string 
will help you much with selecting the correct table on the pascal side. 
You need something like a pascal symbol name, not a string. The closest 
thing you can get is a pointer. You could just take the addresses of 
your pascal maps and push lightuserdata instead of a string name. 
However, the most common solution would probably be to replace your 
`env` table with a pointer-sized full userdata which stores the address 
of a pascal table inside. This address is protected from Lua code, you 
can easily and efficiently access the pascal table from your 
`__index`/`__newindex` functions, and you can share the same metamethods 
and metatable for all your pascal tables you want to export.


> Regards,
> Linuxfan

HTH,
Philipp