Master C++ Variables: Types, Declarations, Naming Rules & Conversions

1. What is a variable?

In C++, a variable is a container for storing data. Each variable has a name (identifier) and a data type that determines what values it can hold and how much memory it occupies.

Variable declaration and initialization

int age = 25; // Declare and initialize an integer variable
float price = 19.99f; // Declare and initialize a single‑precision floating‑point number
char grade = 'A'; // Declare and initialize a character variable
bool isActive = true; // Declare and initialize a boolean variable

Declaration : tells the compiler the variable’s name and type.

Initialization : assigns an initial value to the variable.

2. C++ basic data types

C++ provides several data types, mainly divided into the following categories:

(1) Integer (int)

Used to store whole numbers (no fractional part).

int num = 42; // standard int (usually 4 bytes)
short smallNum = 10; // short int (usually 2 bytes)
long bigNum = 100000L; // long int (4 or 8 bytes)
long long hugeNum = 1'000'000'000L; // long long (usually 8 bytes)

int

range is typically –2,147,483,648 to 2,147,483,647 on a 32‑bit system.

(2) Floating‑point (float, double)

Used to store numbers with a fractional part.

float pi = 3.14f; // single‑precision (≈6‑7 significant digits)
double precisePi = 3.1415926535; // double‑precision (≈15‑16 significant digits)

float

occupies 4 bytes, double occupies 8 bytes.

By default, a literal like 3.14 is a double; adding the suffix f makes it a float.

(3) Character (char)

Used to store a single character (letter, digit, symbol).

char letter = 'A'; // store a character
char symbol = '$'; // store a symbol
char newline = '
'; // escape character for newline

char

occupies 1 byte (8 bits) and can represent ASCII characters (0‑127).

Use single quotes for characters and double quotes for strings, e.g., "Hello".

(4) Boolean (bool)

Used to store logical values true or false.

bool isCppFun = true; // true (1)
bool isHard = false; // false (0)

bool

occupies 1 byte but logically needs only 1 bit.

Commonly used in conditional statements, e.g., if (isCppFun) { … }.

3. Variable naming rules

Names may contain letters, digits, and underscores, but cannot start with a digit.

Names are case‑sensitive (e.g., age vs Age).

Keywords such as int, return, class cannot be used as identifiers.

Recommended styles: camelCase (e.g., userAge) or snake_case (e.g., user_age).

Examples

Valid identifiers:

int userAge;
float accountBalance;
bool isLoggedIn;

Invalid identifiers:

int 123age; // cannot start with a digit
float return; // cannot use a keyword
char first name; // cannot contain spaces

4. Type conversion (Type Conversion)

C++ allows conversion between different data types.

(1) Implicit conversion (automatic)

int num = 10;
double decimal = num; // int → double (automatic)

Converting a smaller type to a larger type (e.g., int → double) is usually safe.

(2) Explicit conversion (cast)

double pi = 3.14159;
int approxPi = (int)pi; // double → int (explicit cast, fractional part lost)

Use (type)variable or C++‑style static_cast<type>(variable) for explicit casts.

5. Summary

int

: stores integers, typically 4 bytes. float: stores single‑precision floating‑point numbers, 4 bytes. double: stores double‑precision floating‑point numbers, 8 bytes. char: stores a single character, 1 byte. bool: stores logical values, 1 byte.