pointer arithmetic example?

[Kerp]

I am still relatively new to doing this even in c, because i was used to using 
vector in c++.

But since i am learning nim i don't know the way to correctly put a pointer 
into a pre allocated pointer using cast in nim.

So here is what i'm working on.
    
    
    import ../obj_parser, streams
    
    
    type
      BinWriter* = ref object of RootObj
       obj: ref obj_data
    
    proc readObjFile*(self:BinWriter,name:string) =
      self.obj = getObjFile(name)
    
    proc createBinary*(self:BinWriter,filename:string) =
       var s = newFileStream(filename,fmwrite);
       if s != nil:
        var total : int = 0
        
        total += (self.obj.vert.len * sizeof(float32))
        total += (self.obj.face.len * sizeof(uint32))
        total += (self.obj.nrml.len * sizeof(float32))
        total += (self.obj.tex.len * sizeof(float32))
        
        var all : pointer = nil
        var vert = alloc(self.obj.vert.len * sizeof(float32))
        for i in 0 .. <self.obj.vert.len:
         vert = cast[pointer](cast[int](self.obj.vert) + i)
         echo("sizeof vert: ",repr(vert))
        
        var face = alloc(self.obj.face.len * sizeof(uint32))
        for i in 0 .. <self.obj.face.len:
         face = cast[pointer](cast[int](self.obj.face) + i)
         echo("sizeof face: ",repr(face))
        
        var nrml = alloc(self.obj.nrml.len * sizeof(float32))
        for i in 0 .. <self.obj.nrml.len:
         nrml = cast[pointer](cast[int](self.obj.nrml) + i)
         echo("sizeof nrml: ",repr(nrml))
        
        var tex = alloc(self.obj.tex.len * sizeof(float32))
        for i in 0 .. <self.obj.tex.len:
         tex = cast[pointer](cast[int](self.obj.tex) + i)
         echo("sizeof tex: ",repr(tex))
        
        #all = cast[all](all + vert + face + nrml + tex) dont know what to do 
here???
        
        dealloc(tex)
        dealloc(nrml)
        dealloc(face)
        dealloc(vert)
        #dealloc(all)
    
    var binary = BinWriter()
    binary.readObjFile("../u.obj")
    binary.createBinary("u.bin")
    

so i checked around but saw no explanation or example of how to do this but 
i'll keep looking.