C++: Variables and Basic Data Types

Last updated: 2026-08-26

In Lesson 02 we set up the development environment and can compile and run programs.

But programs can't just output fixed text — they need to be able to remember things and process data. That's what variables are for.

Just like in math, you need to define x, y, z first; in programming, you must "declare" variables before you can store data.

1. What is a Variable?

(1) 1.1 Variables in Everyday Life

A variable is a "box" in memory — you can put things in it (assign values) and take things out (read values).

(2) 1.2 Why Do We Need Variables?

Without variables, programs can only do "rigid" things:

// Without variables, you can only output fixed text
std::cout << "Hello, World!" << std::endl;

With variables, programs come "alive":

// With variables, you can remember user input
std::string name;
std::cin >> name;
std::cout << "Hello, " << name << "!" << std::endl;

2. Basic Data Types

C++ is a statically typed language — every variable must have its type declared first, and the type cannot change.

(1) 2.1 Five Basic Data Types

💡 Tip: string is not a "basic type" in C++, but a type provided by the standard library. You need #include <string> before using it.

(2) 2.2 Declaring Variables

Syntax:

Type variableName;

Example:

int age; // Declare an integer variable named age
double price; // Declare a floating-point variable named price
char grade; // Declare a character variable named grade
bool isPassed; // Declare a boolean variable named isPassed
std::string name; // Declare a string variable named name

(3) 2.3 Declaring and Assigning at the Same Time (Initialization)

Syntax:

Type variableName = value;

Example:

int age = 25; // Declare age and assign 25
double price = 19.99; // Declare price and assign 19.99
char grade = 'A'; // Declare grade and assign 'A' (note: characters use single quotes)
bool isPassed = true; // Declare isPassed and assign true
std::string name = "MOTO"; // Declare name and assign "MOTO" (note: strings use double quotes)

💡 Tip: C++11 introduced a more modern initialization style (uniform initialization):

int age = {25}; // Uniform initialization (recommended)
double price{19.99}; // Can omit the equals sign (recommended)

This style prevents narrowing conversions (e.g., assigning 3.14 to int will cause an error instead of silently truncating).

3. Using Variables

▶ Example 1: Calculate the Area of a Circle (Difficulty ⭐)

#include <iostream>

int main() {
 const double PI = 3.14159; // Constant (covered later)
 
 double radius;
 std::cout << "Please enter the radius of the circle：";
 std::cin >> radius;
 
 double area = PI * radius * radius;
 std::cout << "The area of the circle is：" << area << std::endl;
 
 return 0;
}

Output:

Please enter the radius of the circle：
The area of the circle is：

Run result:

Please enter the radius of the circle：5
The area of the circle is：78.5397

Code breakdown:

1. const double PI = 3.14159; — Declare a constant PI whose value cannot change
2. double radius; — Declare a floating-point variable radius
3. std::cin >> radius; — Read user input from the keyboard and store it in radius
4. double area = PI * radius * radius; — Calculate the area and store the result in area
5. std::cout << ... — Output the result

4. Variable Naming Rules

C++ has strict rules for variable names:

(1) 4.1 Hard Rules (Must Follow)

C++ Keyword List (partial):

int, double, char, bool, string, if, else, for, while, 
return, class, public, private, new, delete, ...

(2) 4.2 Soft Rules (Strongly Recommended)

The compiler doesn't enforce these, but following them makes your code more readable:

💡 Tip: Different teams may have different naming conventions. This tutorial uses camelCase.

5. Constants (const)

Some values should never change, such as π, the number of days in a year, or the maximum length of an array.

(1) 5.1 Why Do We Need Constants?

Without constants (bad example):

double circumference = 3.14159 * diameter; // This 3.14159 is a "magic number"
double area = 3.14159 * radius * radius; // If π is wrong, you have to fix it in two places

With constants (good example):

const double PI = 3.14159;
double circumference = PI * diameter; // Clear and easy to change
double area = PI * radius * radius;

(2) 5.2 How to Declare Constants

Syntax:

const Type CONST_NAME = value;

Example:

const int DAYS_PER_WEEK = 7;
const double PI = 3.14159;
const std::string SCHOOL_NAME = "Meijia Vocational College";

💡 Tip: Constant names are typically ALL_CAPS with underscores separating words — this is a C++ convention.

(3) 5.3 Characteristics of Constants

1. Must be initialized (must be assigned a value when declared)

const int MAX_SIZE; // ❌ Error: constant must be initialized

2. Cannot be modified

const int MAX_SIZE = 100;
MAX_SIZE = 200; // ❌ Error: cannot modify a constant's value

6. Input and Output (cin and cout Advanced)

(1) 6.1 Reading Multiple Variables at Once

cin can read multiple variables in one statement, separated by spaces or newlines:

#include <iostream>
#include <string>

int main() {
 int age;
 std::string name;
 
 std::cout << "Please enter your name and age（separated by spaces）：";
 std::cin >> name >> age; // Read two variables at once
 
 std::cout << "Hello，" << name << "! You are " << age << " years old。" << std::endl;
 return 0;
}

Run result:

Please enter your name and age（separated by spaces）：MOTO 35
Hello，MOTO! You are 35 years old。

(2) 6.2 Mixing cin and cout

#include <iostream>
#include <string>

int main() {
 std::string name;
 int age;
 double height;
 
 std::cout << "Please enter your name: ";
 std::cin >> name;
 
 std::cout << "Please enter your age: ";
 std::cin >> age;
 
 std::cout << "Please enter your height (meters): ";
 std::cin >> height;
 
 std::cout << "========== Personal Info ==========" << std::endl;
 std::cout << "Name: " << name << std::endl;
 std::cout << "Age: " << age << " years old" << std::endl;
 std::cout << "Height: " << height << " meters" << std::endl;
 
 return 0;
}

7. Common Errors and Debugging

(1) 7.1 Forgetting to Initialize a Variable

Error example:

#include <iostream>

int main() {
 int x; // Declared x but didn't assign a value
 std::cout << x << std::endl; // ❌ x's value is "garbage" — undefined
 return 0;
}

Correct approach:

int x = 0; // Initialize when declaring
std::cout << x << std::endl; // ✅ Outputs 0

💡 Tip: Modern C++ compilers will warn you about using uninitialized variables, but don't rely on the compiler — develop the good habit of always initializing.

(2) 7.2 Type Mismatch

Error example:

int age;
std::cin >> age;
// If the user enters "abc", age will remain unchanged (or worse)

Correct approach (we'll learn how to validate input later):

int age;
std::cin >> age;
if (std::cin.fail()) {
 std::cout << "InputError！Please enter a number." << std::endl;
}

(This topic will be covered in detail in Lesson 08 — for now, just be aware of the issue.)

8. Practice: Simple Calculator

▶ Example 2: Addition, Subtraction, Multiplication, Division Calculator (Difficulty ⭐⭐)

#include <iostream>

int main() {
 double num1, num2;
 char op;
 
 std::cout << "Please enter the first number: ";
 std::cin >> num1;
 
 std::cout << "Please enter an operator (+ - * /): ";
 std::cin >> op;
 
 std::cout << "Please enter the second number: ";
 std::cin >> num2;
 
 double result;
 if (op == '+') {
 result = num1 + num2;
 } else if (op == '-') {
 result = num1 - num2;
 } else if (op == '*') {
 result = num1 * num2;
 } else if (op == '/') {
 if (num2 != 0) {
 result = num1 / num2;
 } else {
 std::cout << "Error: Divisor cannot be 0!" << std::endl;
 return 1; // Abnormal exit
 }
 } else {
 std::cout << "Error: Unsupported operator!" << std::endl;
 return 1;
 }
 
 std::cout << "Result: " << num1 << " " << op << " " << num2 << " = " << result << std::endl;
 return 0;
}

Output:

Please enter the first number: 
Please enter an operator (+ - * /): 
Please enter the second number: 
Error: Divisor cannot be 0!
Error: Unsupported operator!
Result:    = 

Run result:

Please enter the first number: 10
Please enter an operator (+ - * /): *
Please enter the second number: 5
Result: 10 * 5 = 50

💡 Tip: This program uses if-else (covered in detail in Lesson 06) — you can copy it for now and understand the general approach.

❓ FAQ

▶ Example 3: Type Conversion (Difficulty ⭐)

#include <iostream>

int main() {
    int a = 5;
    double b = 3.7;

    // Implicit conversion
    double c = a + b;
    std::cout << "Implicit conversion: " << c << std::endl;

    // Explicit conversion (C-style)
    int d = (int)b;
    std::cout << "C-style cast: " << d << std::endl;

    // C++-style conversion
    int e = static_cast<int>(b);
    std::cout << "C++-style cast: " << e << std::endl;

    return 0;
}

Output:

Implicit conversion: 
C-style cast: 
C++-style cast: 

• Variables are "boxes" in memory used to store data
• C++ has 5 basic data types: int, double, char, bool, string
• Declaring a variable: Type variableName;
• Initializing a variable: Type variableName = value; (uniform initialization Type variableName{value}; is recommended)
• Constants are declared with const, their values cannot change, and they must be initialized
• Use cin for input, cout for output
• Variable names should be meaningful and follow naming rules

📖 Summary

• Basic types: int, double, char, bool
• Type conversion: static_cast, dynamic_cast
• auto: automatic type deduction
• const: constant, cannot be modified

📝 Exercises

1. Basic (Difficulty ⭐): Write a program that declares three variables: name (string), age (integer), score (floating-point), assigns values to each, and outputs them.

My name is Alice，this year 20 years old，FinalScore 92.5 points。

2. Intermediate (Difficulty ⭐⭐): Write a program that lets the user input the radius of a circle, then calculates and outputs the circumference and area (circumference = 2πr, area = πr²).

3. Define π as a constant (3.14159)

4. Output with 2 decimal places (hint: use std::fixed and std::setprecision, requires #include <iomanip>)

5. Challenge (Difficulty ⭐⭐⭐): Write a program that converts Fahrenheit to Celsius:

6. Let the user input a Fahrenheit temperature

7. Use the formula C = (F - 32) × 5/9 to calculate Celsius

8. Output the result (1 decimal place)

9. Example: input 68, output "68°F = 20.0°C"

• Variables are data storage units in memory; they must be declared before use

1. C++ basic types: int (integer), double (floating-point), char (character), bool (boolean), string (string)

• Variable initialization methods: = assignment / uniform initialization {}
• Constants are declared with const; their values cannot be modified and must be initialized
• Naming rules: letters/digits/underscores, cannot start with a digit, case-sensitive

13. 🚀 Next Step

Now that you've learned about variables and data types, next we'll learn C++ operators (Lesson 04) — enabling variables to perform arithmetic, comparisons, and logical judgments.

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