"Classes" and "Constructors" in Lua?

Monome
1

Stumbling across some frustrating issues trying to make a script more object-oriented. I’m trying to construct a hierarchy of sorts, but putting “constructors” inside others is resulting in empty tables.

local Subdivisions = {}
function Subdivisions:new(n)
  o = {}
  self.__index = self
  setmetatable(o, self)
  for i=1, n do
    o[i] = {on = true, StepParams = {}}
  end
  return o
end

local Beats = {}
function Beats:new(n)
  o = {}
  self.__index = self
  setmetatable(o, self)
  for i=1, n do
    o[i] = {on = true, subs = Subdivisions:new(1)} 
  end
  return o
end

I’m expecting the line >> b = Beats:new(3) to look like

  • 1 => { on = true, subs = table: 0x… }
  • 2 => { on = true, subs = table: 0x… }
  • 3 => { on = true, subs = table: 0x… }

and each subs would look like

  • 1 => { on = true, StepParams = {} }

Instead, tab.print(b) results in what b.subs should be:

  • 1 => table: 0x… = { on = true, StepParams = {} }

replacing the :new() functions with default tables fixes the structure, but then I have to build the subdivisions externally and change them. I’m just trying to figure out the source of this problem.

2

variables in lua are global by default, even in functions. nasty surprise if you’re coming from python.

so i think o in the inner construcor is wiping out o in the outer constructor.

this works for me:


Subdivisions = {}
function Subdivisions:new(n)
  local o = {}
  self.__index = self
  setmetatable(o, self)
  for i=1,n do
     o[i] = {on = true, StepParams = {}}
  end
  return o
end

Beats = {}
function Beats:new(n, m)
  local o = {}
  self.__index = self
  setmetatable(o, self)
  for i=1, n do
    o[i] = {on = true, subs = Subdivisions:new(m)} 
  end
  return o
end

local b = Beats:new(10, 5)
print(b[1].subs, #b[1].subs)                            -- a table with 5 entires
print(b[1].subs[1].StepParams, #b[1].subs[1].StepParams)   -- a table with no entries
  • made o local in both c-tors
  • changed count of subdivisions in Beats constructor for illustrative purposes
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