Variables and Data Types in C++

Giving your program labelled boxes to remember things

Big Idea: A variable is a named box in the computer's memory where your program keeps a value. In C++ every box has a type that says what kind of value it holds — a whole number, a decimal, a single letter, a yes/no, or some text.

By the end you will be able to:

declare a variable with a type and a name,
use the five everyday types int, double, char, bool and std::string,
explain why C++ needs the type written up front,
give a variable a starting value (initialization).

1. What is a variable?

Think of your computer's memory as a big shelf full of boxes. A variable is one box that you give a name so you can find it again, and inside it you store a value.

The name lets you read the value back or change it later. Here is the smallest useful example — one box called score holding the number 10:

#include <iostream>

int main() {
    int score = 10;
    std::cout << "Score: " << score << "\n";
    return 0;
}

Output:

Score: 10

int score = 10; reads as: "make an int box named score and put 10 in it." std::cout then prints what is inside.

2. Why C++ wants the type up front

Every box needs a type written before its name. The type is a promise about what will live inside. C++ is a statically typed language: you tell it the type once, at declaration, and it stays that type.

Why bother? Two big reasons:

Size & shape. The computer needs to know how much memory to set aside. A whole number, a letter, and a decimal are stored differently.
Safety. If you promise a box holds a whole number, the compiler can warn you before the program runs if you try to misuse it. Mistakes are caught early.

Each variable is a labelled box: a name, a value inside it, and a type that says what kind of value.

An analogy: variables are like labelled jars in a kitchen. The label (type) says "sugar only" or "salt only". You would not pour soup into the sugar jar — and C++ helps you keep each jar for its own kind of thing.

Variables and Data Types in C++