Re: Checking if a table has changed

[Florian Berger <fberger@...>]

Thanks for the tip! I found code from lua-l archive (see below, I 
modified it a little bit). The problem is that as you can see that it 
does not work for nested tables. I could traverse recursively the 
subtables through and call proxy for each table but I cannot do this 
because of performance reasons. This cannot be automated somehow?

Floru


Source: http://lua-users.org/lists/lua-l/2003-07/msg00173.html

do
  local signature = {}

  -- set a proxy on table
  function proxy(table)
    local meta =
    {
      __index = table,

      __newindex = function(_, index, value)
        print('Modified!')
        table[index] = value
      end,

      __signature = signature,
    }

    return setmetatable({}, meta)
  end

  local n = next

  -- next replacement to support pairs and "old style" for loops
  function next(table, index)
    local m = getmetatable(table)
    return n(m and m.__signature == signature and m.__index or table, 
index)
  end
end

local x = proxy( {{a = 1}, a = 1, b = 2} )

for k, v in pairs(x) do
  print(k, v)
end

x[1].a = 5
x.a = 2


Result:
1       table: 00379EB8
a       1
b       2
Modified!




Jamie Webb wrote:

> On Thursday 11 December 2003 08:22, Florian Berger wrote:
>
> > Hi.
> >
> > Is there a simple and efficient way to check if a table has changed? By
> > using metatables I'm able to catch some modifications but not all of them.
>
>
> You could use a proxy table with a __newindex which makes modifications to the 
> actual table and sets a 'modified' flag. Since the proxy table stays empty, 
> __newindex is always called.
>
>  -- Jamie Webb
>
> There are only 10 types of people in this world:
> those who understand binary, and those who don't.