Re: Problem extending a table

[Peter Cawley <lua@...>]

Imagine for a minute what would happen *if* tables automatically had a
{__index = table} metatable:
* Checks like "if getmetatable(t) == nil then" to see if a table has a
metatable would fail
* Tables which contained members like "sort" and "concat" would cause
code which used the t:sort() syntax to fail
* All tables would have fields like "sort" and "concat" by default,
causing code like "t.sort = t.sort or 'ascending'" to fail

In short, it would cause more problems that it would solve.

On Sat, Feb 14, 2009 at 2:40 PM, Kristofer Karlsson
<kristofer.karlsson@gmail.com> wrote:
> 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.
>>
>
>