Metatable

Metatable are special field could be injected into a table, which could include custom operators and any arbitrary field for special use(implementing OOP for example)

Metatable Method

A table literal does not contain metatable on creation.

• getmetatable(t: table): table: retrieve metatable of a table
• setmetatable(t: table, metatable: table): table: set the metatable to t and returns the altered table reference

Indexing

• __index

  • table: redirects indexing to another table when key is absent from the table.
  • fun(this: table, key: any): any: a custom getter to be called when rawget(this, idx) cannot find one value for the key.

• __newindex

  • fun(this: table, key: any, value: any): custom setter to set the value to key only when the key is absent from the table.

Indexing Proxy

Either __index or __newindex could only be triggered when key is absent in the table, so if you do need to monitor all access to the table, such as monitor value updates, an empty table is required to be the proxy. The proxy must be empty during the whole lifetime, so that each access to it would trigger __index and __newindex, and the real table represents the data would be wrapped as a closed variable inside __newindex and __index.

IMPORTANT

However, the price of proxy table is, you can't iterate it by ipairs or pairs, they don't redirect to __index.

local real = {
  name = 'foo',
}

local proxy = setmetatable({}, {
  __index = function(_, key) return real[key] end,
  __newindex = function(_, key, value)
    if real[key] ~= nil then
      print('updating the value')
      real[key] = value
    else
      print('setting this value for the first time')
      real[key] = value
    end
  end,
})

proxy.name = 'bar' -- updating the value