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> If there were a metamethod called when a table element is deleted (set to
> nil) you could do without the proxy table here (and in many other
> situations).
> Having a __removeindex metamethod is something *I* would like in 5.2 :)

I had to use the proxy userdata to override __len, since you can't set __len
for tables. Otherwise, you could get away with just

function newhash()
  local n = 0
  return setmetatable({}, {
    __index = function(o,k) return rawget(o,k) end,
    __newindex = function(o,k,v)
      if rawget(o,k)==nil then n=n+1 elseif v==nil then n=n-1 end
    __len = function(o) return n end


Lua 5.1  Copyright (C) 1994-2006, PUC-Rio
> require'hash'
> h=newhash()
> h.k1=10
> h.k2=20
> = #h
> = debug.getupvalue(getmetatable(h).__newindex, 1)
n       2

That is, I'm sorry, but I fail to see why one might need a __removeindex
metamethod. OTOH, being able to set a __len metamethod for tables would be
useful (along with a "rawlen" function for tables, probably).


A mathematician is a device for turning coffee into theorems.
        -- P. Erdos 

Luis Carvalho
Applied Math PhD Student - Brown University
PGP Key: E820854A <>