Adding Objects

Part of the Object Oriented Programming section of Coddy's Lua journey — lesson 26 of 70.

You've already seen how metamethods like __tostring and __eq let you customize how objects behave. Now we'll extend this to arithmetic operations, starting with addition.

The __add metamethod defines what happens when you use the + operator between two objects. When Lua sees a + b, it looks for __add in the metatable of either operand.

local Vector = {}
Vector.__index = Vector

function Vector:new(x, y)
    local obj = {x = x, y = y}
    setmetatable(obj, Vector)
    return obj
end

function Vector.__add(v1, v2)
    return Vector:new(v1.x + v2.x, v1.y + v2.y)
end

Notice that __add is defined with a dot, not a colon. It receives both operands as arguments and should return a new object rather than modifying either original. This keeps the operation predictable and safe.

local a = Vector:new(3, 4)
local b = Vector:new(1, 2)
local c = a + b

print(c.x, c.y)  -- Output: 4  6

The key insight is that __add must be in the metatable itself (here, Vector), not just in the object. Since we set Vector as the metatable for all instances, they all share this behavior automatically.

Challenge

Easy

Let's build a Vector class that supports addition using the + operator. This is your first step into operator overloading—making your objects work naturally with Lua's built-in operators.

You'll organize your code across two files:

Vector.lua: Define your Vector class with x and y components. Include a :new(x, y) constructor and implement the __add metamethod so that adding two vectors creates a new vector with the combined components. Remember that __add uses dot syntax (not colon) and receives both operands as arguments.
main.lua: Require your Vector module, create vector instances, and demonstrate that adding them produces the correct result.

You will receive four inputs:

First vector's x component
First vector's y component
Second vector's x component
Second vector's y component

In your main file, create two vectors with the given components, add them together using the + operator, and print the resulting vector's x and y values on separate lines.

For example, if the inputs are 3, 4, 1, 2, the output should be:

4
6

The first vector (3, 4) plus the second vector (1, 2) equals a new vector (4, 6).

Cheat sheet

The __add metamethod defines what happens when you use the + operator between two objects.

When Lua sees a + b, it looks for __add in the metatable of either operand.

local Vector = {}
Vector.__index = Vector

function Vector:new(x, y)
    local obj = {x = x, y = y}
    setmetatable(obj, Vector)
    return obj
end

function Vector.__add(v1, v2)
    return Vector:new(v1.x + v2.x, v1.y + v2.y)
end

__add is defined with a dot, not a colon. It receives both operands as arguments and should return a new object rather than modifying either original.

local a = Vector:new(3, 4)
local b = Vector:new(1, 2)
local c = a + b

print(c.x, c.y)  -- Output: 4  6

__add must be in the metatable itself, not just in the object.

Try it yourself

-- Require the Vector module
local Vector = require('Vector')

-- Read input
local x1 = tonumber(io.read())
local y1 = tonumber(io.read())
local x2 = tonumber(io.read())
local y2 = tonumber(io.read())

-- TODO: Create two Vector instances using the input values

-- TODO: Add the two vectors using the + operator

-- TODO: Print the resulting vector's x and y values on separate lines