Lesson 21 +10 XP

Objects and Variables

Before we write programs that do useful work, we need a way to store information. That's where objects and variables come in.

What is an object?

An object is a piece of memory (in your computer's RAM) that stores a value. You can think of it as a labeled box: the box has space inside, and what we put in the box is the value.

What is a variable?

A variable is an object that has a name we can use to refer to it. Naming the box makes it usable:

int x;   // creates an object named x that can hold an integer

The type (int) says what kind of value the box can hold.
The name (x) is how we talk about the box in our code.
The statement as a whole is a declaration: it tells the compiler "please create this variable".

Why store values?

Almost everything a program does works on data: a game tracks your score, a store keeps its prices, a chatbot remembers your name. Instead of re-typing a value again and again, we store it once in a variable and use the name everywhere.

int score = 10;
score = score + 1;   // add 1 to the stored score
cout << score;       // prints 11

The box starts holding 10, then we update it, then we print it. One name, reused all over the program.

A definition is a declaration

In everyday C++, "declaration" and "definition" are often used to mean the same thing: int x; creates the variable. A variable can be declared many times, but defined only once - the definition is the one that actually creates the box.

TL;DR

An object is a chunk of memory storing a value.
A variable is a named object: a type + a name.
int x; declares a variable named x of type int.
Variables let us store data once and reuse it anywhere in the program.

Objects and Variables