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Jamie Webb wrote:

The issue is most likely one of speed. The current behaviour is invoked only for 'tables misses', which are presumed rare. The behaviour you are suggesting would require an additional lookup for every table access, regardless of whether the table actually has this metamethod. The same problem applies for a __next metamethod, which is one I'd certainly find useful.

This is the impression I'm under as well, but I was wondering: since these entries in the metatable (the metamethods that is) are used by the VM it wouldn't be too ugly to keep a number of flags with each table/userdata for storing wich (of the "standard") metamethods have been set in it's metatable. Then every time, say, an index operation takes place all you have to do is check if the index flag is set (a very cheap operation) instead of looking up the metatable and indexing it. The flags can either be kept with the userdata/tables or their metatables depends on how tables/metatables are handled internally (if it's fast to get a table/userdata's metatable or if it's fast to get a metatable's table/userdata). Of course there are bound to be complications with this (like the fact that a metatable can be bound to more than one tables/userdatas), that I'm unaware of, as I know almost nothing about lua internals but I was just wondering if a solution in that direction might be possible as __settable, __gettable, __next metamethods would be very nice. Proxy tables seem to make lua API C code quite complex (you have to keep references to proxy tables, etc.) and besides it would be more logical to have these metamethods (if they could be implemented cheaply) than not to.

Dimitris P.