Importing class and using its methods accross files

Question

Im trying to save myself some headache in a project im working on by splitting it up in files but I cannot seem to be able to cross call functions. (inside file_a, call a function in file_b which calls a function in file_a)

Is this just not possible or am i doing it wrong?

file_a:

local imported = require("fileb")
m = imported:new(BC)
m.print_message("hi")

file_b:

class_b = {
    BC = {}
}

function class_b:print_message(message)
    print(message)
end

function class_b:new (BC)
    local AB = {} 
    AB = BC or {}
    AB._index = class_b
    setmetatable(AB, self)
    return AB
end
return class_b

When i run this i get: attempt to call nil value (field 'print_message')

My project is much larger and complex at the moment but i extracted the logic i am working with right now to show what im trying to do to be able to cross call the functions.

This has nothing to do with files and Lua doesn't have classes. `m` does get to be a new table `{}` with the metatable set to `class_b`. But, what part of that means it should have a `print_message` function? Nothing gives `m` a `print_message` function, since it's not in the table `m` itself and there's no `__index` either. — user253751, Sep 29 '22 at 19:02
How could i go about being able to cross use functions? Right now pretty much im trying to have class_b add itself to BC which is passed as an argument to new() — jmelger, Sep 29 '22 at 19:14
Lua doesn't look in the metatable the way you're thinking - it looks in `__index` in the metatable. So if you want it to find functions in `class_b` then you set the metatable to some table whose `__index` is `class_b` — user253751, Sep 29 '22 at 19:16
I edited how i've got it right now. Its still not working tho, could you elaborate? — jmelger, Sep 29 '22 at 19:23
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/248455/discussion-between-jmelger-and-user253751). — jmelger, Sep 29 '22 at 19:34
this is a trivial error that could be avoided by carefully reading any online example on basic oop in Lua. please read the Lua manual and make sure you understand what __index does as you obviously don't know it. — Piglet, Sep 30 '22 at 06:00

score 0 · Answer 1 · answered Sep 30 '22 at 05:59

You're calling m["print_message"] which is a nil value. Calling nil values is not allowed as it doesn't make any sense to call a nil value.

If you index a field that does not exist in a table, Lua will check wether the table has a metatable and if that metatable has a field __index.

While that field exists in m it does not exist in its metatable class_b.

class_b.__index must refer to class_b to make this work.

Importing class and using its methods accross files

1 Answers1