Advanced Metatables: Operator Overloading & __call

Fundamentals covered __index for method lookup. Metatables support many more metamethods that let a table respond to operators, comparisons, tostring, and even being called like a function.

Operator overloading

Lua

local Vector = {}
Vector.__index = Vector

function Vector.new(x, y)
  return setmetatable({ x = x, y = y }, Vector)
end

Vector.__add = function(a, b)
  return Vector.new(a.x + b.x, a.y + b.y)
end

Vector.__tostring = function(v)
  return string.format("(%g, %g)", v.x, v.y)
end

Vector.__eq = function(a, b)
  return a.x == b.x and a.y == b.y
end

local v1 = Vector.new(1, 2)
local v2 = Vector.new(3, 4)

print(v1 + v2)          -- (4, 6), uses __add, then __tostring for print
print(v1 == Vector.new(1, 2)) -- true, uses __eq

local Vector = {}
Vector.__index = Vector

function Vector.new(x, y)
  return setmetatable({ x = x, y = y }, Vector)
end

Vector.__add = function(a, b)
  return Vector.new(a.x + b.x, a.y + b.y)
end

Vector.__tostring = function(v)
  return string.format("(%g, %g)", v.x, v.y)
end

Vector.__eq = function(a, b)
  return a.x == b.x and a.y == b.y
end

local v1 = Vector.new(1, 2)
local v2 = Vector.new(3, 4)

print(v1 + v2)          -- (4, 6), uses __add, then __tostring for print
print(v1 == Vector.new(1, 2)) -- true, uses __eq

print(v1 + v2) runs __add to build a new Vector, then print calls tostring on it, which invokes __tostring. Without __tostring, printing a table falls back to Lua's default table: 0x... representation.

__call: making a table callable

Lua

local Multiplier = setmetatable({ factor = 3 }, {
  __call = function(self, n)
    return n * self.factor
  end,
})

print(Multiplier(10)) -- 30, calling the table like a function

local Multiplier = setmetatable({ factor = 3 }, {
  __call = function(self, n)
    return n * self.factor
  end,
})

print(Multiplier(10)) -- 30, calling the table like a function

__call is what lets some libraries expose an object that's both a table of methods/config and directly invokable.

__newindex: intercepting new keys

Lua

local readOnly = setmetatable({ pi = 3.14159 }, {
  __newindex = function(_, key, _)
    error("cannot set '" .. key .. "', this table is read-only")
  end,
})

print(readOnly.pi) -- 3.14159, ordinary reads still work
readOnly.pi = 4     -- errors: cannot set 'pi', this table is read-only

local readOnly = setmetatable({ pi = 3.14159 }, {
  __newindex = function(_, key, _)
    error("cannot set '" .. key .. "', this table is read-only")
  end,
})

print(readOnly.pi) -- 3.14159, ordinary reads still work
readOnly.pi = 4     -- errors: cannot set 'pi', this table is read-only

__newindex only fires when you assign to a key that doesn't already exist in the table, it's the standard trick for building read-only tables, or a proxy that validates/logs writes.

NOTE

There's a matching, easy-to-miss rule: __index (as a function) only fires on a missing-key read, and __newindex only fires on a missing-key write. Once a key genuinely exists on the table itself, both metamethods are bypassed entirely for that key.

Advanced Metatables: Operator Overloading & __call

Operator overloading

__call: making a table callable

__newindex: intercepting new keys

📝 Advanced Metatables Quiz