Re: Problem extending a table

[Kristofer Karlsson <kristofer.karlsson@...>]

The difference is that strings share a metatable with __index set to the string table as default.
This is not true for generic tables, since individual tables can be set to different metatables.
If you want, you can do:
setmetatable(a, {__index = table}) and get the behaviour you wanted for table a

On Sat, Feb 14, 2009 at 3:35 PM, Peter Hickman <peterhi@ntlworld.com> wrote:

I can see what you are both saying but I came from code like this:

string.fred = function (s)

return s..s

end

a = "xxx"

print(a)

b = a:fred()

print(b)

Which does work. It has added a method to the string object and an instance of a string has picked up the method. I thought that it would also work for tables also

On 14 Feb 2009, at 14:08, Duncan Cross wrote:

On Sat, Feb 14, 2009 at 2:02 PM, Peter Hickman <peterhi@ntlworld.com> wrote:

I have some code that I want to use to add an invert method to tables.

table.invert = function (t)
       local i = {}
       for k in pairs(t) do
               i[t[k]] = true
       end
       return i
end

When I call it as "table.invert(a)" it works fine, but it errors when I call it as "a:invert()"

lua: invert.lua:15: attempt to call method 'invert' (a nil value)

My understanding was that a:invert() was just sugar on table.invert(a), what am I doing wrong?

a:invert() is actually sugar for a.invert(a), which will only be equivalent to table.invert(a) if you have a metatable set on 'a' which looks up the 'table' library when __index is called.

The situation is different with strings because all string values share a common metatable, while tables each individually have their own metatable, which by default is nil. Therefore tables will never inherit any methods as soon as they are created, something has to be done to provide them.