Re: Question about __index and classes

[Lorenzo Donati <lorenzodonatibz@...>]

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