Re: Question about __index and classes

[HyperHacker <hyperhacker@...>]

On Sat, Aug 27, 2011 at 18:39, Lorenzo Donati
<lorenzodonatibz@interfree.it> wrote:
> On 28/08/2011 0.52, Michael Kirsch wrote:
>>
>> There is something I don't understand about the way __index and
>> metamethods work. The way I understand, the "." operator looks in the
>> table itself and the ":" operator looks directly in the metatable. If
>> so, then why is it necessary to say:
>>
>>     MyClass.__index = MyClass
>>
>> for you to be able to use ":"?
>>
>> If ":" looks right in the metatable, then it shouldn't be needed. To
>> me it seems like what it would do is let you call metamethods using
>> the "." operator, but it doesn't work that way for some reason.
>>
>>
>>
> I won't repeat what others have correctly said about "." and ":".
>
> As for your doubt regarding:
>
> MyClass.__index = MyClass   -- (*)
>
> this is only an idiom for certain OO approaches.
>
> In this approach you model a class as a table:
>
> MyClass = {}
>
> then add methods to it:
>
> function MyClass.Method1( self, arg1, arg2 )
> ...
> end
>
>
> Then you create objects as tables having MyClass as metatable:
>
>
> obj1 = setmetatable( {}, MyClass )
>
> the problem is that for this syntax to work:
>
> obj1:Method1( 'foo', 'bar' )
>
> which is completely equivalent to:
>
> obj1.Method1( obj1, 'foo', 'bar' )  -- (1)
>
> Method1 must be retrieved and called. From (1) is evident that the lookup is
> done in obj1, but obj1 hasn't got such a key.
>
> Metatable magic begins here: when the lookup for Method1 in (1) fails, Lua
> notices that obj1 has MyClass as metatable. Then searches the metatable for
> a specially named key: __index
>
> If MyClass has such a key and that key points to a table, then the lookup in
> (1) (which failed) will be redone in the table pointed to by __index.
>
>
> That table could be ANY table, but it is idiomatic to make it point to
> MyClass using (*), so that the lookup will find a key named "Method1" and
> perform the call in (1) using the function pointed to by Method1.
>
>
> Remember that Lua has no notion of function names; function in Lua are first
> class values; "Method1" is not the name of a function inside "MyClass", but
> it is the name of a key whose value happens to be a function.
>
> Moreover, bear in mind that setting a metatable on a table, doesn't make it
> automatically an object of some class.
>
>
> The OO terminology is a useful abstraction over all this stuff, but remember
> that Lua has no "true classes" and this is only one of the many approaches
> to OO [1].
>
> Hope this helps.
>
>
> [1] http://lua-users.org/wiki/ObjectOrientedProgramming
>
>
> -- Lorenzo
>
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It sounds like the OP is describing how . and : worked in Lua 4? A lot
has changed since then, might want to read over the manual for a newer
version...

-- 
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