RE: Hashing userdata

["Tom Miles" <Tom@...>]

You and Jerome have hit the nail on the head.  They are userdata created
with different lua_newuserdata calls.  I hadn't considered the
posibility that they would get seperate hash values if they contain the
same data, this seems a little odd being as two seperate strings get
hashed to the same value if they have the same value, but I'll let it
slide :)

> -----Original Message-----
> From: lua-bounces@bazar2.conectiva.com.br 
> [mailto:lua-bounces@bazar2.conectiva.com.br] On Behalf Of 
> Matt Campbell
> Sent: 25 January 2008 16:28
> To: Lua list
> Subject: Re: Hashing userdata
> 
> OK, are both "key" and "v.key" references to the same Lua userdata? 
> That is, do they both come from the same invocation of 
> lua_newuserdata?
> 
> I'm guessing the answer is no.  What you need is a table, 
> probably only referenced in the Lua registry, which maps C++ 
> pointers (as Lua light
> userdata) to the corresponding full userdata.  This table 
> should have weak values; that is, its metatable's "__mode" 
> field should be set to "v".  Then, whenever your C++ code 
> needs to send an object to Lua, it should first look up the 
> object's pointer (again, as light userdata) in this cache 
> table.  If a corresponding full userdata object is found, the 
> C++ code should send that to Lua.  Otherwise, it should create a new
> full userdata object and map that to the C++ pointer in the 
> cache table before sending it to Lua.  Then there will be 
> only one Lua userdata object for each C++ object.
> 
> Matt
> 


 
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