Help with metatables inheritance

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#1

Thought it would be fun to try and expand my knowledge with metatables, and in doing so I’ve attempted to create an inventory system. There is a BaseSlot, PurchasedSlot, HotbarSlot, and then multiple other slots for different item types.

  • BaseSlot is the. base… Every slot inherits this.
  • PurchasedSlot is created once said slot is purchased, upgrading a BaseSlot to a PurchasedSlot.
  • HotbarSlot inherits a PurchasedSlot since you start out with them.

Now this is where I am having trouble… When an item is added to a slot, the slot changes depending on the item type (ie. ToolSlot). Now this could inherit either a PurchasedSlot or a HotbarSlot depending which is the next available slot. I get how to inherit from a predetermined parent, but i’m not sure how to be able to do so on the fly (ie. On creation).

function ToolSlot.new(purchasedSlot) -- purchasedSlot can be either a PurchaseSlot or HotbarSlot
    -- This was my latest attempt

    -- ToolSlot.__index = purchasedSlot   -- If a method does exist, look it up in super
    -- local self = setmetatable(purchasedSlot, ToolSlot)   -- Set metatable to ToolSlot
    
    -- Errors with 'loop in gettable'

    return self
end

Thanks for any help!

#2

I think you’d want to make a factory instead of a .new. But, whatever works.

function ToolSlot.new(purchasedSlot)
    local self = {}
    self.__index = purchasedSlot
    return setmetatable(self, self)
end

However, I think the bigger problem is that you have too many kinds of slots. You should make a generic slot instead that can contain any items. Then, you can add or remove slots based off how much slots a player should have. You’d probably have lists of slot for where they can be. So, inventory would be a list of slots and so would the hotbar.

#3

Thanks for the reply! Also good points, however, continuing onwards just to solve this problem… What if ToolSlot also had functions that need to be called?

function ToolSlot:Talk()
    ...
end

Obviously that would error if called by a created ToolSlot object. How would I go about solving that?
Guessing I would somehow have to check if it exists in ToolSlot first, if not __index purchasedSlot?

#4

I think I have solved it actually

function ToolSlot.new(purchasedSlot)
    local self = {}
    setmetatable(self, {
        __index = ToolSlot
    })
    setmetatable(ToolSlot, {
        __index = purchasedSlot
    })

    return self
end
#5

You can’t set multiple metatables, you are essentially resetting the metatable. I realize what’s actually needed is a Mixin. The easiest way of doing that would be with a function. So, you’d essentially want to check in two places if there is the key you are looking for.

function ToolSlot.new(purchasedSlot)
    return setmetatable({}, {
        __index = function(tbl, key) return ToolSlot[key] or purchasedSlot[key] end
    }
end

Now, keep in mind that the ToolSlot is going to override any key that is found in purchasedSlot (if it doesn’t already exist in the new object of ToolSlot). I think this isn’t an ideal solution since you are now creating a unique metatable for each object. The true problem is at an architectural level as mentioned before – but if it works, it works :stuck_out_tongue: .

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