Formatting Strings

Part of the Object Oriented Programming section of Coddy's Lua journey — lesson 16 of 70.

When you print an object directly, Lua shows something unhelpful like table: 0x55a...—just a memory address. The __tostring metamethod lets you define exactly what string represents your object when it's printed or converted to text.

In the class pattern, you add __tostring as a function in your class table. Since the class itself serves as the metatable for instances, Lua automatically finds and uses this function:

local Player = {}
Player.__index = Player

function Player:new(name, level)
    local instance = {}
    setmetatable(instance, self)
    instance.name = name
    instance.level = level
    return instance
end

function Player:__tostring()
    return "Player: " .. self.name .. " (Level " .. self.level .. ")"
end

Now when you print a Player instance, Lua calls your __tostring method automatically:

local hero = Player:new("Luna", 5)
print(hero)  -- Output: Player: Luna (Level 5)

Notice that __tostring uses the colon syntax and receives self, giving you access to all instance data. The function must return a string—you can format it however makes sense for your object, combining any attributes into a readable representation.

Challenge

Easy

Let's build a Book class that knows how to introduce itself! When you print a book directly, Lua will display a nicely formatted string instead of a cryptic memory address—all thanks to the __tostring metamethod.

You'll organize your code across two files:

Book.lua: Define your Book class with the standard prototype pattern. Each book should store a title and an author. Include a :new(title, author) constructor and implement the __tostring metamethod so that printing a book displays its information in a readable format.
main.lua: Require your Book module, create book instances, and print them directly to see your custom string formatting in action.

Your __tostring method should return a string in this exact format:

"[title]" by [author]

Notice the title is wrapped in double quotes within the output string.

You will receive four inputs:

Title of the first book
Author of the first book
Title of the second book
Author of the second book

In your main file, create two books using the provided inputs and print each book directly (not by calling a method—just print(book)). Each book should appear on its own line.

For example, if the inputs are The Hobbit, J.R.R. Tolkien, 1984, and George Orwell, the output should be:

"The Hobbit" by J.R.R. Tolkien
"1984" by George Orwell

Remember that __tostring must return a string, and since your class table serves as the metatable for instances, Lua will automatically find and use your __tostring function when you print a book.

Cheat sheet

The __tostring metamethod defines how an object is represented as a string when printed or converted to text.

Without __tostring, printing an object shows only its memory address:

print(someObject)  -- Output: table: 0x55a...

Add __tostring as a method in your class to customize the string representation:

local Player = {}
Player.__index = Player

function Player:new(name, level)
    local instance = {}
    setmetatable(instance, self)
    instance.name = name
    instance.level = level
    return instance
end

function Player:__tostring()
    return "Player: " .. self.name .. " (Level " .. self.level .. ")"
end

When you print an instance, Lua automatically calls __tostring:

local hero = Player:new("Luna", 5)
print(hero)  -- Output: Player: Luna (Level 5)

Key points:

Use colon syntax (:) so the method receives self
Must return a string
Access instance data through self
Since the class table serves as the metatable, Lua finds __tostring automatically

Try it yourself

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

-- Read inputs
local title1 = io.read()
local author1 = io.read()
local title2 = io.read()
local author2 = io.read()

-- TODO: Create two book instances using Book:new()


-- TODO: Print each book directly (just use print(book))