RE: Catch get/set of userdata in table?

["Michael Flad" <Michael@...>]

> -----Original Message-----
> From: owner-lua-l@tecgraf.puc-rio.br
> [mailto:owner-lua-l@tecgraf.puc-rio.br]On Behalf Of J. Perkins
> Sent: Thursday, September 06, 2001 23:34
> To: Multiple recipients of list
> Subject: Catch get/set of userdata in table?
> 
> 
> Is there a way to catch a get/set to a userdata object that is stored in 
> a table? I know how to do it for a global variable, like LHF showed a 
> while back (see example below) -- is there some way to do an equivalent
> operation on a table member?
> 
> I have a C++/Lua component system, and rather than mirroring all of the 
> get...() and set...() calls of C++ I'd like to make them look like 
> regular table variables and have assignments transparently call the C++ 
> get...() and set...() methods under the hood. So if I had something like 
> this in C++:
> 
> struct MyComponent {
>     int getValue();
>     void setValue (int value);
> };
> 
> 
> In Lua it would look like:
> 
>     MyComponent.value = 22;
>     x = MyComponent.value
> 
> I got it working by *not* putting anything in the table, catching the 
> "gettable" and "settable" calls and manually looking up the proper table 
> method from an internal array of entry points. It's ugly. Is there a 
> more elegant way to make this happen?


You could always check if it's a userdata of a type you want to
handle by yourself and in every other case, call the rawset/rawget
methods - in this case you could use the tables as usual and you
only need the special handling for your userdata.

That's the way i do it in a first implementation.
I think somtimes later i may optimize this by adding another type of
tag method, something like "setuserdata"/"getuserdata". After a
very quick look at the sources of Lua it seems easy to implement.

I think for tables, you could insert the required code in 

void luaV_settable (lua_State *L, StkId t, StkId key)

I haven't done any tests right now, but i let you know when i've
done the changes (don't have the time right now).


Regards,
Michael Flad

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