setmetatable()

The setmetatable() function is used to set or change the metatable for a given table in Lua. Metatables are tables that define how operations on another table behave. By setting a metatable, you can customize table behavior for operations like addition, indexing, and method calls.

Syntax

setmetatable(table, metatable)

Parameters

Return

Returns the table that was passed as the first parameter.

Description

Using setmetatable(), you can define custom behaviors for table operations by utilizing fields in the metatable such as __index, __newindex, and others. This allows for powerful object-oriented programming techniques in Lua.

Examples

Setting a simple metatable

local myTable = {}
local myMetatable = {
  __index = function(t, key)
    return "Key " .. key .. " not found!"
  end
}

setmetatable(myTable, myMetatable)

print(myTable.someKey)  -- Output: Key someKey not found!

Customizing table addition

local vector1 = { x = 1, y = 2 }
local vector2 = { x = 3, y = 4 }

setmetatable(vector1, {
  __add = function(v1, v2)
    return { x = v1.x + v2.x, y = v1.y + v2.y }
  end
})

local result = vector1 + vector2
print(result.x, result.y)  -- Output: 4 6

See also

• getmetatable(): Retrieves the metatable for a given table.
• rawset(): Sets a value in a table without invoking the metatable's __newindex.
• rawget(): Retrieves a value from a table without invoking the metatable's __index.