Re: I don't understand this application of metamethod

[Tim Mensch <tim-lua-l@...>]

On 5/24/2012 10:25 PM, Emeka wrote:
> What is the essence of the below?
>
> local Request = {}
> Request.__index = Request

The above doesn't do anything interesting by itself, though if Request is later assigned as the metatable for another table, AND the Request table had another member, it would be relevant:

local aRequest = {}
setmetatable( aRequest, Request ); -- Request is now the metatable of the table referred to by "aRequest"

function Request:someFunction()
    print("a function on Request")
end

aRequest:someFunction()

The __index metamethod tells Lua what to do if someone tries to look up an undefined key on a table. If __index is a function, it gets called. If __index is a table, then it looks for the key in that table. So when Lua can't find a member on aRequest, it tries the __index metamethod, sees that it's a table, finds the key "someFunction" on that table, and then tries to call it as a function.

Tim