Acknowledgements: These slides are adapted from slide set 10 for the course Programming in Lua created by Fabio Mascarenhas, Federal University of Rio de Janeiro, Brazil. It was presented at Nankai University, P. R. China, in July 2013.

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Metatable

A metatable modifies behavior of another table

• enable use of arithmetic, concatenation, relational operators

• override behavior of ==, ~=, and # operators

• override behavior of builtins tostring, pairs, ipairs

• give values for missing fields, intercept creation of new fields

• enable calling table as function

Scope of Metatables

• Each table can have own metatable -- own behavior

• Several tables can share a metatable -- similar behaviors

• setmetatable changes metatable of table, returns table

• getmetatable returns metatable of table (nil if none)

• Bad style: Changing metatable after assigning it to table (gives poor performance)

Metamethods (1)

• Set metamethods to specify effect of metatable

• Associate function with specially named field

• Lua 5.2 has 19 metamethods -- names begin with two underscores

  __add, __sub, __mul, __div, __mod, __pow, __unm, 
  __concat, __len, __eq, __lt, __le, __index, __newindex, 
  __call, __tostring, __ipairs, __pairs, __gc 

• Lua 5.3 deprecates __ipairs, adds new integer/bit metamethods

Metamethods (2)

• Set functions as metamethod values

• Can also use tables for __index, and __newindex

• Implement single inheritance using table for __index

Complex Number Example

Augment earlier complex number module with operations:

• Addition of complex and real numbers

• Structural comparison for equality

• Modulus with #

• Pretty-printing with tostring

• Source file complex.lua

Sharing a Metatable

• Create table private to module as metatable for each complex number

  local mt = {} 
  
  local function new(r, i) 
     return setmetatable({ real = r or 0, im = i or 0 }, mt) 
  end 

• Define query accessor for complex numbers

  local function is_complex(v) 
      return getmetatable(v) == mt 
  end 

Overloading + with __add (1)

• Recall previously defined complex.add function

  local function add(c1, c2) 
      return new(c1.real + c2.real, c1.im + c2.im) 
  end 

• Define metamethod __add to reference add, now + works for complex

  mt.__add = add 
  
  > c1 = complex.new(2, 3) 
  > c2 = complex.new(1, 5)
  > print(complex.tostring(c1 + c2)) 
  3+8i 

Overloading + with __add (2)

• Try adding real to complex

  > c3 = c1 + 5 
  .\complex.lua:20: attempt to index local 'c2' (a number value) 
  stack traceback: 
          .\complex.lua:20: in function '__add' 
          stdin:1: in main chunk 
         [C]: in ? 

• What is happening?

  Calls __add metamethod of complex number!

Arithmetic Resolution (1)

• If left operand has __add metamethod, Lua calls it -- modify complex.add

  local function add(c1, c2) 
      if not is_complex(c2) then
          return new(c1.real + c2, c1.im) 
      end 
      return new(c1.real + c2.real, c1.im + c2.im) 
  end 

• Now adding real to complex works:

  > c1 = complex.new(2, 3) 
  > c3 = c1 + 5 
  > print(complex.tostring(c3)) 
  7+3i 

Arithmetic Resolution (2)

• What about adding a complex to a real?

  > c3 = 5 + c1 
  .\complex.lua:20: attempt to index local 'c1' (a number value) 
  stack traceback: 
          .\complex.lua:20: in function '__add' 
          stdin:1: in main chunk 
          [C]: in ? 

• What is happening?

  Again calls __add metamethod of complex number!

Arithmetic Resolution (3)

• If left operand has no __add metamethod but second has, Lua calls it -- modify complex.add

  local function add(c1, c2) 
      if not is_complex(c1) then
          return new(c2.real + c1, c2.im)
      end
      if not is_complex(c2) then 
          return new(c1.real + c2, c1.im) 
      end 
      return new(c1.real + c2.real, c1.im + c2.im) 
  end 

• Now adding complex to real works

  > c3 = 5 + c1 
  > print(complex.tostring(c3)) 
  7+3i

Equality

• Metamethod __eq controls both == and ~=

• Slightly different rule from arithmetic -- calls metamethod if both operands have same metatable

  local function eq(c1, c2)
      return (c1.real == c2.real) and (c1.im == c2.im)
  end
  mt.__eq = eq 
  
  > c1 = complex.new(1, 2)
  > c2 = complex.new(2, 3) 
  > c3 = complex.new(3, 5)
  > print(c1 + c2 == c3) 
  true 
  > print(c1 ~= c2) 
  true 

• Complex/real comparison false even if imaginary part 0

Overloading # and tostring

• Metamethods __len and __tostring work similarly -- receive table, return result

  local function modulus(c) 
      return math.sqrt(c.real * c.real + c.im * c.im) 
  end 
  mt.__len = modulus 
  
  local function tos(c) 
      return tostring(c.real) .. "+" .. tostring(c.im) .. "i" 
  end 
  mt.__tostring = tos 
  
  > c1 = complex.new(3, 4) 
  > print(#c1) 
  5
  > print(tostring(c1))
  3+4i 
  > print(c1)    -- print uses tostring
  3+4i 

Relational Operations (1)

• Metamethod __lt for < works like arithmetic metamethods

  For > Lua uses __lt with operands reversed

• Metamethod __le for <= works like arithmetic metamethods

  But <= uses __lt if __le not available, with operands reversed and negating

• Why two metamethods? -- for partial orders

Relational Operations (2)

    local function le(c1, c2)
        if not is_complex(c1) then 
            return (c1 <= c2.real) and (c2.im >= 0)
        end 
        if not is_complex(c2) then 
            return (c1.real <= c2) and (c2.im <= 0) 
        end 
        return (c1.real <= c2.real) and (c1.im <= c2.im)
    end
    
    mt.__le = le
            
    local function lt(c1, c2) 
        return c1 <= c2 and not (c2 <= c1) 
    end

    mt.__lt = lt 

Using __index and __newindex

• Calls __index metamethod when indexing with undefined key -- pass table and key -- return result

• Calls __newindex metamethod when assigning to undefined key -- passes table, key, and value

• Use with empty table as proxy for another table -- catch indexing and/or assignment operations, delegate to other table

• Both __index and __newindex can be tables instead of functions -- redo indexing operation on the table

A Counting Proxy

    local mt = {}                        > proxy = require "proxy" 
                                         > t = proxy.track({}) 
    function mt.__index(t, k)            > t.foo = 5 
        t.__READS = t.__READS + 1        > print(t.foo)
        return t.__TABLE[k]              5 
    end                                  > t.foo = 2 
                                         > print(t.__READS, t.__WRITES)
    function mt.__newindex(t, k, v)      1       2 
        t.__WRITES = t.__WRITES + 1 
        t.__TABLE[k] = v 
    end 

    local function track(t) 
        local proxy = { __TABLE = t, __READS = 0, __WRITES = 0} 
        return setmetatable(proxy, mt) 
    end 

    return { track = track } 

Quiz

We can try to work around the limitation of __eq so we can have complex.new(2,0) == 2 by making complex.new return a real if the imaginary part is 0.

Which operations will continue to work with this change, and which will not work anymore?

Quiz Solution

We try to work around the limitation of __eq so we can have complex.new(2,0) == 2 by making complex.new return a real if the imaginary part is 0.

Which operations will continue to work with this change, and which will not work anymore?

It breaks #.