Re: [LuaJIT FFI] Issues with cdata/ctype

[Duncan Cross <duncan.cross@...>]

On Sun, May 29, 2011 at 4:26 PM, Justin Cormack
<justin@specialbusservice.com> wrote:
>> A. Determining 'one of a set of special cdata types' (2). I have tried
>> using tostring(ffi.typeof(value)) and looking this up in a table, and
>> it seems to work, but should I trust this method to be future-proof?
> No thats not the recommended method, you should use ffi.istype(type, val)
> which returns true if it is that type.

Oh, I forgot about that, oops.

It does mean I need to go through a (possibly large) array of types
and check each one rather than looking up a single identifying token
once though, which doesn't seem ideal, but JIT can probably help with
that for all I know.

> You could in addition support duck typing, ie also
> accept anything that has a ptr and len method as well.

That's true. cdatas will throw an error if you try to index them to
look to see if a method exists, though, but I could put it in a pcall.

>> Also, it strikes me that stage (2) would not be necessary if it were
>> possible to apply (4) to these cdata values instead of special-casing
>> them. I know there are heavy restrictions on the metatables of cdata,
>> and that is understandable. But I'm wondering if it would be possible
>> to call custom metamethods on cdata with something like a
>> ffi.metacall(cdata, '__mymetamethod')?
>
> Why cant you just add a __tobuffer metamethod to all the types?

getmetatable(cdata) always seems to return 'ffi', currently.

-Duncan