how to import classes from a file (middleclass)

[HappyYoyo09]

I'm using the middleclass library, and I have my classes and my main.lua file separated. I'm trying to figure out how to import classes from the objects.lua file to the main.lua file.

this is my main.lua script, I'm new to lua, so I'm not really sure how importing things with require works

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local class = require "middleclass"
local objects = require "objects"

updateOrder = {} --every object in this array will have its :update(delta) function called in love.update(delta)
drawOrder = {} --similar to update order, but for drawing. its function is :draw(), and it's called in love.draw


local player = Player:new(0,0) --this is where the error occurs, it doesn't have the Player class since I'm not sure how to import the classes from the objects file yet.

player:add()

function love.update(delta)
    for i=1,#updateOrder,1 do
        updateOrder[i]:update(delta)
    end
    
end

the code works when it isn't separated into two separate files, this is the error I get when i try to run the code

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Error

main.lua:8: attempt to index global 'Player' (a nil value)


Traceback

[love "callbacks.lua"]:228: in function 'handler'
main.lua:8: in main chunk
[C]: in function 'require'
[C]: in function 'xpcall'
[C]: in function 'xpcall'

[lost-in-code]

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local player = objects.Player:new(0,0)

Maybe ?

[pgimeno]

Depends on what objects.lua returns. The answer by lost-in-code will work if it returns a table with all the objects.

You have to understand that no matter the class library you're using, they will work the same way with respect to class and self passing:

- Colon syntax is a Lua thing, which is what implicitly passes a `self` parameter to methods. All class libraries will use it.
- Classes themselves are tables, and you need to make these tables available to the program somehow if you want to use them. So you need to pass them like you would any other value.

So, in the case of a library file, if you are using local variables for the classes (highly recommended), you need to return them at the end of the library, so that when you use require() in main, the return value of the require function itself contains them.

So, for example, if you have a library that returns 4, you would do this in the library:

four.lua:

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    return 4

main.lua:

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local four = require('four')
print(four)  -- prints 4

It may sound like a stupid example, but the fact is that with tables, and hence with classes, it's the same thing. If you have a library that implements a single class:
player.lua:

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require [[ your class library of choice ]]
local Player = [[ definition of Player using your library of choice ]]
[[ methods for Player ]]

return Player  -- return it to the main program

and when you require it from main, you read the returned value:
main.lua:

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local Player = require('player')
local player = Player:new(0, 0)

But in your case you have multiple classes in the same file, so you need to return them all. You would do that like you do with any other values.

You could do this, for example:

objects.lua:

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local Player = [[ definition of the player class ]]
local Enemy = [[ definition of the enemy class ]]
return {Player = Player, Enemy = Enemy}

main.lua:

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local objects = require('objects')
local player = objects.Player:new(0, 0)
...

That's one way to do it, but not the only one. You could also do this:
objects.lua:

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local Player = [[ definition of player ]]
local Enemy = [[ definition of enemy ]]
return {Player, Enemy}

and in main.lua:

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local Player, Enemy = unpack(require('objects'))

Or whatever other means. Of course you could also declare the classes as global variables in objects.lua, and then they will be available throughout your program without you needing to even require the objects from each source file that uses them. I don't recommend this but maybe it's the simplest for you.