Attempt to index field (nil value)

Question

Whenever I try to use Body.pos, it always says that it is a nil value. But it is assigned a value in the new function, so it should not be nil.

My code:

Vector={
    x=nil,y=nil

    ,new=function (self,x,y)
        o={}
        setmetatable(o,self)
        self.__index=self

        o.x=x or 1
        o.y=y or 1

        return o
    end

    -- utility functions here
}

Body={
    pos=nil
    ,vel=nil
    ,acc=nil
    ,mass=nil

    ,new=function (self,pos,vel,acc,mass)
        o={}
        setmetatable(o,self)
        self.__index=self

        o.pos=pos or Vector:new()
        o.vel=vel or Vector:new()
        o.acc=acc or Vector:new()
        o.mass=mass or 1

        return o
    end

    ,applyForce=function (self,v)
        self.acc:add(v:scale(1/self.mass))
    end

    ,applyGravity=function (self)
        self.acc:add(GRAVITY_VECTOR)
    end

    ,step=function (self)
        self.vel:add(self.acc)
        self.pos:add(self.vel)

        self.acc:scale(0)
    end
}

Trial code:

b=Body:new()
print(b.pos.x) -- shows error that pos is nil

Vector:new() does not return nil, but still Body.pos is always nil. I don't know what I am doing wrong here.

EDIT: Added Vector implementation

Answer 1

The issue is your OOP implementation. The metatable o needs to be set as a local variable.

In your code o is a global variable hence why you always reset it when you create a new object.

Vector={
    -- ... 
    ,new=function (self,x,y)
        local o={}
        -- ...
    end
    -- ... 
}

Body={
    -- ... 
    ,new=function (self,pos,vel,acc,mass)
        local o={}
        -- ...
        return o
    end
    -- ... 
}

b=Body:new()
print(b.pos.x) -- 1

Basically any variable that isn't declared as local is automatically put into the global scope.

Additionally I recommend using a linter (e.g. luacheck) to automatically detect issues like this.