Lua: Metatables

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This article assumes the use of Lua 5.1. Information may not be accurate or may need revision if you are using a different version.

Metatables are a form of table that may be attached to any other table, including itself.

An individual table may only have one metatable attached, but that metatable may have its own metatable, and this may be cascaded indefinitely. An individual metatable may be attached to any number of tables, however.

The primary purpose of metatables is to allow tables to be added together, an operation that is by default impossible.

table1 = {1, 2, 3}
table2 = {10, 20, 30}
table3 = table1 + table2 -- Returns an error

Because tables may contain any number of separate non-nil types of data, adding tables is by default impossible, and will always return an error, due to the overall impracticality of doing so.

To allow tables to be added together, the metamethod __add must be defined within a metatable, and that metatable must then be appended to a regular table:

Example code

function newTable(t_data)
  return setmetatable(t_data, mt) -- Assigns the same metatable to the table passed to this function
end

metatable = { 
    __add = function (a, b)
        local sum = { } -- This is the new table we will be returning.
        for i = 1, math.max(#a, #b) do -- Runs as long as entries exist in the larger table
            sum[i] = (a[i] or 0) + (b[i] or 0) -- Adds one table entry to its correspondent, using 0 for entries that don't exist
        end
        return newTable(sum) -- Set the metatable or we can't use this table for future addition operations!
    end,

    __tostring = function (t)
        return table.concat(t, " ") -- Control how this table is coerced into a string
    end,
}

table1 = newTable {1, 2, 3}
table2 = newTable {10, 20, 30}
table3 = table1 + table2 -- valid after much headache

print(table3) -- Was it worth it?