lua metatable methods cheatsheet.md

A metatable in Lua defines various extraneous behaviors for a table when indexed, modified, interacted with, etc. They are Lua's core metaprogramming feature; most well known for being useful to emulate classes much like an OOP language.

Any table (and userdata) may be assigned a metatable. You can define a metatable for a table as such:

-- Our sample table
local tab = {}
-- Our metatable
local metatable = {
  -- This table is then what holds the metamethods or metafields
  -- that Lua will read to determine the table's behavior, eg.
  __index = function() --[[ ... ]] end
}

setmetatable(tab, metatable)

The metatable can be retrieved back (unless the __metatable metafield is set):

local metatable = getmetatable(tab) -- Returns nil if tab has no metatable set

Afterwards, you may remove a metatable and restore regular functionality as such:

setmetatable(tab, nil)

Here are the metamethods that you can define, and their behavior:

Operators

In an argument signature like f(a, b), a and b don't necessarily have to be instances of your metatable. One of them will always be, but not necessarily the first. Beware!

Calculation operators

If any operand for an operation is not a number, Lua will try to call a metamethod. It starts by checking the first operand (even if it is a number); if that operand does not define a metamethod for the operation, then Lua will check the second operand. If Lua can find a metamethod, it calls the metamethod with the two operands as arguments, and the result of the call (adjusted to one value) is the result of the operation. Otherwise, if no metamethod is found, Lua raises an error.

__add(a, b): the addition (+) operation.
__sub(a, b): the subtraction (-) operation.
__mul(a, b): the multiplication (*) operation.
__div(a, b): the division (/) operation.
__unm(a): the negation (unary -) operation.
__mod(a, b) (Lua 5.1): the modulo (%) operation.
__pow(a, b) (Lua 5.1): the exponentiation (^) operation.
__idiv(a, b) (Lua 5.3): the floor division (//) operation.

Bitwise operators (Lua 5.3)

Behavior similar to calculation operators, except that Lua will try a metamethod if any operand is neither an integer nor a float coercible to an integer (see §3.4.3).

__band(a, b): the bitwise AND (&) operation.
__bor(a, b): the bitwise OR (|) operation.
__bxor(a, b): the bitwise exclusive OR (binary ~) operation.
__bnot(a): the bitwise NOT (unary ~) operation.
__shl(a, b): the bitwise left shift (<<) operation.
__shr(a): the bitwise right shift (>>) operation.

Equation operators

__eq(a, b): the equal (==) operation. Behavior similar to calculation operators, except that Lua will try a metamethod only when the values being compared are either both tables or both full userdata and they are not primitively equal. The result of the call is always converted to a boolean.
__lt(a, b): the less than (<) operation. Behavior similar to calculation operators, except that Lua will try a metamethod only when the values being compared are neither both numbers nor both strings. Moreover, the result of the call is always converted to a boolean.
__le(a, b): the less equal (<=) operation. Behavior similar to the less than operation.

Misc operators

__concat(a, b): the concatenation (..) operation. Behavior similar to calculation operators, except that Lua will try a metamethod if any operand is neither a string nor a number (which is always coercible to a string).
__len(a) (Lua 5.1): the length (#) operation. If the object is not a string, Lua will try its metamethod. If there is a metamethod, Lua calls it with the object as argument, and the result of the call (always adjusted to one value) is the result of the operation. If there is no metamethod but the object is a table, then Lua uses the table length operation (see §3.4.7). Otherwise, Lua raises an error.

Behavioral methods

Indexing

__index: The indexing access operation table[key]. This event happens when table is not a table or when key is not present in table. The metavalue is looked up in the metatable of table. The metavalue for this event can be either a function, a table, or any value with an __index metavalue. If it is a function, it is called with table and key as arguments, and the result of the call (adjusted to one value) is the result of the operation. Otherwise, the final result is the result of indexing this metavalue with key. This indexing is regular, not raw, and therefore can trigger another __index metavalue. Examples:
```
local data = {foo = 'bar'}
local proxy = setmetatable({}, {__index = data})

print(proxy.foo) --> 'bar'
```
```
local tab = setmetatable({count = 0}, {
  __index = function(self, _k)
    self.count = self.count + 1
    return self.count
  end
})

print(tab.indextest) --> 1
print(tab.indextestagain) --> 2
print(tab.asdfasdf) --> 3
print(tab[1234]) --> 4
```
__newindex: The indexing assignment table[key] = value. Like the index event, this event happens when table is not a table or when key is not present in table. The metavalue is looked up in the metatable of table. Like with indexing, the metavalue for this event can be either a function, a table, or any value with an __newindex metavalue. If it is a function, it is called with table, key, and value as arguments. Otherwise, Lua repeats the indexing assignment over this metavalue with the same key and value. This assignment is regular, not raw, and therefore can trigger another __newindex metavalue. Whenever a __newindex metavalue is invoked, Lua does not perform the primitive assignment. If needed, the metamethod itself can call rawset to do the assignment. Examples:
```
local t = setmetatable({}, {
  __newindex = function(t, key, value)
    if type(value) == 'number' then
      rawset(t, key, value * value)
    else
      rawset(t, key, value)
    end
  end
})

t.foo = 'foo'
t.bar = 4
t.la = 10
print(t.foo) --> 'foo'
print(t.bar) --> 16
print(t.la) --> 100
```

Calling

__call(args): The call operation func(args). This event happens when Lua tries to call a non-function value (that is, func is not a function). The metamethod is looked up in func. If present, the metamethod is called with func as its first argument, followed by the arguments of the original call (args). All results of the call are the results of the operation. This is the only metamethod that allows multiple results.

Garbage collection & memory management

__mode: Controls how "weak" a table is. If present, must be one of the following strings: "k", for a table with weak keys; "v", for a table with weak values; or "kv", for a table with both weak keys and values. A table with weak keys and strong values is also called an ephemeron table. In an ephemeron table, a value is considered reachable only if its key is reachable. In particular, if the only reference to a key comes through its value, the pair is removed. (see §2.5.4)
__close(value, err?) (Lua 5.4): Called when a variable is closed (see §3.3.8).
__gc(): Called when the the garbage collector detects that the corresponding table or userdata is dead. See §2.5.3

Misc

__tostring(): If the metatable of v has a __tostring field, then tostring calls the corresponding value with v as the argument, and uses the result of the call as its result. Otherwise, if the metatable of v has a __name field with a string value, tostring may use that string in its final result.
__metatable: Changes the behavior of getmetatable. If object does not have a metatable, returns nil. Otherwise, if the object's metatable has a __metatable field, returns the associated value. Otherwise, returns the metatable of the given object.
__name: Used very rarely internally (see luaL_newmetatable) and by tostring() if the __tostring metamethod is missing.
__pairs() (Lua 5.2): Affects iteration when using the pairs() function, letting you define a custom iterator function (see pairs(), next()).
__ipairs() (Lua 5.2 - Lua 5.3): Same as __pairs(), except for the ipairs() function. Deprecated as of Lua 5.4

Implementation-specific

__iterator() (MoonSharp): Called if the argument f of a for ... in ... loop is not actually a function.
__usedindex() (SRB2): Called when assigning an index/value to an existing key/variable.