Initializing Attributes

Part of the Object Oriented Programming section of Coddy's Lua journey — lesson 9 of 70.

The :new() constructor you learned creates objects, but every instance starts identical. Real objects need unique data—a person has a specific name, a player has a starting score. To make objects useful, the constructor must accept arguments and assign them to the new instance.

Here's how to expand the constructor to accept initial values:

local Dog = {}
Dog.__index = Dog

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

function Dog:introduce()
    print("I am " .. self.name .. ", a " .. self.breed)
end

The constructor now takes name and breed as parameters. After creating the empty instance and setting its metatable, we assign these values directly to the instance's fields. Each object created will have its own name and breed.

local dog1 = Dog:new("Rex", "German Shepherd")
local dog2 = Dog:new("Bella", "Poodle")

dog1:introduce()  -- Output: I am Rex, a German Shepherd
dog2:introduce()  -- Output: I am Bella, a Poodle

Both dogs share the same :introduce() method from the prototype, but each has its own unique name and breed stored in its instance table. This is the power of combining prototypes with parameterized constructors—shared behavior with individual state.

Challenge

Easy

Let's build a Person class that creates unique individuals with their own names and ages. You'll organize your code across two files to practice proper module structure.

You'll create:

Person.lua: Define your Person class with a constructor that accepts a name and age. Include a :introduce() method that lets each person introduce themselves.
main.lua: Require your Person module and create a person instance using the provided inputs. Then have them introduce themselves.

Your :new(name, age) constructor should accept both values and store them on the new instance. The :introduce() method should print in this exact format:

Hello, my name is [name] and I am [age] years old

You will receive two inputs:

The person's name
The person's age

For example, if the inputs are Alice and 25, the output should be:

Hello, my name is Alice and I am 25 years old

Cheat sheet

To create objects with unique data, expand the :new() constructor to accept parameters and assign them to the instance:

local Dog = {}
Dog.__index = Dog

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

function Dog:introduce()
    print("I am " .. self.name .. ", a " .. self.breed)
end

Create instances with different values:

local dog1 = Dog:new("Rex", "German Shepherd")
local dog2 = Dog:new("Bella", "Poodle")

dog1:introduce()  -- Output: I am Rex, a German Shepherd
dog2:introduce()  -- Output: I am Bella, a Poodle

Each object has its own unique data stored in its instance table, while sharing methods from the prototype.

Try it yourself

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

-- Read input
local name = io.read()
local age = io.read()

-- TODO: Create a new Person instance using Person:new(name, age)

-- TODO: Call the introduce() method on your person instance