Variables and Data Types

C++ is statically typed: every variable has a type that is fixed when it is declared and checked by the compiler before your program ever runs.

Declaring Variables

A variable declaration reserves memory of a specific size and gives it a name. You can declare and assign in one step, or declare first and assign later. The compiler will reject any attempt to store a value of the wrong type in a variable.

int age = 25;
double price = 19.99;
char grade = 'A';
bool isEnrolled = true;
std::string name = "Thabo";

std::cout << name << " is " << age << " years old." << std::endl;

Common Built-In Types

Choosing the right type matters for both correctness and memory efficiency. The table below covers the types you will use in almost every program.

• int — whole numbers, typically 4 bytes (e.g. -2, 0, 42).
• double — decimal numbers with high precision (e.g. 3.14159).
• float — decimal numbers with less precision, uses less memory than double.
• char — a single character, stored in 1 byte (e.g. 'A').
• bool — true or false only.
• std::string — text of any length (requires #include <string>).

Constants

Use const when a value should never change after it is set. This protects against accidental reassignment and communicates intent to anyone reading your code.

const double PI = 3.14159;
const int MAX_STUDENTS = 30;
// PI = 3.2; // Compile error — cannot reassign a const

Common Mistakes

• Trying to store a decimal value in an int, which silently truncates the fractional part.
• Forgetting to #include <string> before using std::string.
• Using single quotes for a multi-character value — 'AB' is invalid for char, which holds exactly one character.
• Naming variables with reserved keywords like int or class.