Metamethods Tutorial

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This is a brief introduction to the concepts of Lua metamethods.

Metamethods

Lua has a powerful extension mechanism which allows you to overload certain operations on Lua objects. Each overloaded object has a metatable of function metamethods associated with it; these are called when appropriate, similar to the concept of operator overloading from many other languages.

A metatable is a regular Lua table containing a set of metamethods, which are associated with events in Lua. Events occur when Lua executes certain operations, like addition, string concatenation, comparisons etc. Metamethods are regular Lua functions which are called when a specific event occurs. The events have names like "add" and "concat" (see manual section 2.8) which correspond with string keys in the metatable like "__add" and "__concat". In this case to add (+) or concatenate (..) two Lua objects.

Metatables

We use the function setmetatable() to make a table act as a metatable for a certain object.

local x = {value = 5}

local mt = {
  __add = function (lhs, rhs) -- "add" event handler
    return { value = lhs.value + rhs.value }
  end
}

setmetatable(x, mt) -- use "mt" as the metatable for "x"

local y = x + x

print(y.value) --> 10

local z = y + y -- error, y doesn't have our metatable. this can be fixed by setting the metatable of the new object inside the metamethod

When the addition operator finds that its operands aren't numbers, it tries checking if one of them has a metatable with an __add key. In this case it does, so it runs the function stored under that key in the metatable, equivalent to this:

local y = (getmetatable(x).__add(x, x)) -- x + x

Metatables are still triggered with math operators if one of the operands is a number. And the left operand is always the first parameter to the function, and the right operand is always the second. This means that the table that has the metamethod might not necessarily be the first parameter to the metamethod.

More events

The following are notes on other of the metamethod events that Lua handles. For a full list of metamethod events, see: MetatableEvents.

__index

This is a very commonly used and versatile metamethod, it lets you run a custom function or use a "fallback" table if a key in a table doesn't exist. If a function is used, its first parameter will be the table that the lookup failed on, and the second parameter will be the key. If a fallback table is used, remember that it can trigger an __index metamethod on it if it has one, so you can create long chains of fallback tables.

local func_example = setmetatable({}, {__index = function (t, k)
  return "key doesn't exist"
end})

local fallback_tbl = setmetatable({
  foo = "bar",
  [123] = 456,
}, {__index=func_example})

local fallback_example = setmetatable({}, {__index=fallback_tbl})

print(func_example[1]) --> key doesn't exist
print(fallback_example.foo) --> bar
print(fallback_example[123]) --> 456
print(fallback_example[456]) --> key doesn't exist

__newindex

This metamethod is called when you try to assign to a key in a table, and that key doesn't exist (contains nil). If the key exists, the metamethod is not triggered.

local t = {}

local m = setmetatable({}, {__newindex = function (table, key, value)
  t[key] = value
end})

m[123] = 456
print(m[123]) --> nil
print(t[123]) --> 456

Comparison operators

__eq is called when the == operator is used on two tables, the reference equality check failed, and both tables have the same __eq metamethod (!).

__lt is called to check if one object is "less than" another. Unline __eq, it's not an error if the two objects have different __lt metamethods, the one on the left will be used.

That's all you need for all of the comparison operators to work with your object. But there will be some cases where both __lt and __eq will need to be called by the same operator. To avoid this, you can optionally add the __le (less than or equal to) metamethod. Now only one of the metamethods will be called with any of the comparison operators.

For example, we can improve the example at the top of the page like this:

local mt
mt = {
  __add = function (lhs, rhs)
    return setmetatable({value = lhs.value + rhs.value}, mt)
  end,
  __eq = function (lhs, rhs)
    return lhs.value == rhs.value
  end,
  __lt = function (lhs, rhs)
    return lhs.value < rhs.value
  end,
  __le = function (lhs, rhs) -- not really necessary, just improves "<=" and ">" performance
    return lhs.value <= rhs.value
  end,
}

__metatable

__metatable is for protecting metatables. If you do not want a program to change the contents of a metatable, you set its __metatable field. With that, the program cannot access the metatable (and therefore cannot change it).

Metamethod manual

See [1] for a list of all metamethods and explanations of what they do.

See Also


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Last edited June 24, 2013 3:38 pm GMT (diff)