what does table.__index = table do when doing OOP?
can anyone explain what table.__index = table does, I have seen alot of people use it when creating custom classes with OOP, but I have no idea what it actually does. I know what the __index metamethod does but I really dont know why they'd use it like that.
like this
local a = {}
a.__index = a
I have seen this tutorial https://devforum.roblox.com/t/all-about-object-oriented-programming/8585 but it doesnt explain well of what it does.
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1 answer
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Question
what does
table.__index = tabledo when doing OOP?
Answer
The same thing when not doing OOP.
Why do people do this?
When creating classes, people set the newly created object's metatable to the original table containing the construction constructor (usually `.new)
Why do this? Well you save on having to redefine new functions every class creation.
Example
local class = {} --// new table for class
class.__index = class --// have class be our metatable too by setting a metamethod to it
function class.new(...) --// our `.new` constructor for creating objects
local newObject = {...} --// define array with all indexes being arguments passed to `.new`
setmetatable(newObject, class) --// set metatable
return newObject --// return our new object for OOP usage
end
function class:myMethod() --// custom method
return #self --// return length of self
end
local myNewObject = class.new("a", "b", "c") --// new object from class's `.new` constructor
print(myNewObject:myMethod()) --// print's 3 because length of table is `3`
Why does this example work?
Well in the example, you may see that we are using self and may be saying "but that just references back to our class variable!" and then probably "why is it printing '3'? Well in metatables though self is set to the owner of the metatable, in this case newObject is the owner of the metatable . (cite line 5) This is also why you would want to use table.__index = table since without setting the metatable, self will just reference back to the original class table.
Should you do this?
Up to you, but I suggest using it because its my preference and its cooler. There might be other advantages i'm not covering, try seeing if anyone else answers!
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