How to make a simple Constructor in lua

[Aconeron]

Hi guys, I'm here to ask what I have to do to make a constructor in lua.
I know it's a dumb question but I searched on the Internet and there's nothing that fits well for me.
Also I come from js where things wheren't so difficult

[Beelz]

Bare minimum could be something along the lines of:

Code: Select all

function constructor(x, y)
	return {
		x = x or math.random()*100,
		y = y or math.random()*100
	}
end

-- Then create instances
local objects = {}
for i = 1, 10 do
	objects[i] = constructor()
	print(i, objects[i].x, objects[i].y)
end

[Davidobot]

While what the above poster is certainly a solution, a more elegant one can be achieved with metatables, and more specifically, the __index part of metatables.
There is what is called a "prototype". They are exactly what you ate looking for - default values for a table.

EDIT:
Example usage:

Code: Select all

planet = {}

planet.proto = {
    x = 0,
    y = 0,
    r = 15,
    m = 1,
    t = 0,

    hasAtmosphere = true,
    speed = 50,
}

planet.mt = {
    __index = planet.proto
}

function planet.new(vars)
    local set = {}
    setmetatable(set, planet.mt)

    for k, v in pairs(vars) do
        set[k] = v
    end

    return set
end

This way you can pass only the variables you need to planet.new. Also make sure not to add the sets to the table where the metatable is stored.

Usage:

Code: Select all

local planet1 = planet.new({x = 30, m = 2})
print(planet1.hasAtomosphere) -> true
print(planet1.x) -> 30
print(planet1.speed) -> 50

[raidho36]

I just do it like this.

Code: Select all

local Class = setmetatable ( { }, { __call = function ( class, ... ) return class.new ( ... ) end } )
Class.__index = Class
Class.__call = function ( self ) return Class.new ( self ) end

function Class.new ( other )
	local self = setmetatable ( { }, Class )
	self.var = other and other.var or 0
	return self
end

So if you call class as a function, it produces new instance. And if you call instance as a function, it produces its clone.

[Aconeron]

Thanks everyone! I didn't excpect some much answers!