C++ Variables: Declaration

Chapter 1: Variables in C++

Variables are one of the most fundamental concepts in programming. A variable is a named storage location in memory that holds data. The value stored in a variable can change while the program is running.

Think of a variable as a labeled container. You can store information inside it, read it later, or update it whenever needed.

For example:

• Store a person's age
• Store a student's name
• Store a product's price
• Store whether a user is logged in

Without variables, it would be impossible to create dynamic and interactive programs.

What is a Variable?

A variable reserves space in your computer's memory and associates that space with a name.

For example:

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1int age = 20;

In this example:

• int is the data type.
• age is the variable name.
• 20 is the value stored in the variable.

Memory representation:

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1Variable Name      Value
2+-----------+    +------+
3|   age     | -> |  20  |
4+-----------+    +------+

Declaring Variables

Before using a variable, you must declare it.

Syntax

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1data_type variable_name;

Example:

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1int age;
2float salary;
3char grade;
4bool isPassed;

Here, memory is allocated, but the variables do not yet have meaningful values.

Initializing Variables

Initialization means assigning a value when the variable is declared.

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1int age = 22;
2float salary = 35000.50;
3char grade = 'A';
4bool isPassed = true;

You can also assign a value later:

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1int age;
2
3age = 22;

Multiple Variable Declaration

You can declare multiple variables of the same type in one statement.

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1int x, y, z;

Initialization example:

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1int a = 10, b = 20, c = 30;

Printing Variables

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1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    int age = 22;
8
9    cout << age;
10
11    return 0;
12}

Output

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122

Printing Multiple Variables

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1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    string name = "Ankit";
8    int age = 22;
9
10    cout << "Name: " << name << endl;
11    cout << "Age: " << age;
12
13    return 0;
14}

Output

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1Name: Ankit
2Age: 22

Changing Variable Values

Variables can be updated during program execution.

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1int age = 20;
2
3age = 21;
4
5cout << age;

Output:

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121

Variable Naming Rules

Choosing meaningful variable names improves code readability.

A valid variable name:

• Can contain letters, digits, and underscores (_)
• Must begin with a letter or underscore
• Cannot begin with a number
• Cannot contain spaces
• Cannot use special characters like @, #, %, $
• Cannot use C++ keywords

Valid Variable Names

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1int age;
2int studentAge;
3int total_marks;
4int number1;
5float averageScore;

Invalid Variable Names

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1int 1age;          // Starts with a number
2int total marks;   // Contains a space
3int class;         // Keyword
4int user-name;     // Hyphen not allowed
5int @price;        // Special character

Naming Conventions

Use descriptive names.

Good:

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1int studentAge;
2double accountBalance;
3bool isLoggedIn;

Poor:

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1int a;
2int x;
3double d;

Use camelCase for variables.

Example:

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1int totalMarks;
2float accountBalance;
3bool isStudent;

C++ Keywords

Keywords are reserved words that have predefined meanings in C++. They cannot be used as variable names.

Examples:

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1int
2float
3double
4char
5bool
6if
7else
8for
9while
10switch
11return
12class
13public
14private
15virtual
16const
17namespace
18using
19new
20delete

Incorrect:

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1int return = 5;

Correct:

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1int returnValue = 5;

Literals in C++

A literal is a fixed value written directly into your code.

Integer Literal

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1100

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1int marks = 100;

Floating-Point Literal

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199.95

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1float price = 99.95;

Character Literal

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1'A'

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1char grade = 'A';

String Literal

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1"Hello"

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1string message = "Welcome";

Boolean Literal

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1true
2false

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1bool status = true;

Constants

A constant is a value that cannot be changed after it has been initialized.

Using const

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1const double PI = 3.14159;

Attempting to modify it:

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1PI = 3.14;

Produces a compilation error.

Example

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1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    const double PI = 3.14159;
8
9    cout << PI;
10
11    return 0;
12}

Output:

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13.14159

constexpr

constexpr creates compile-time constants.

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1constexpr int DAYS_IN_WEEK = 7;

Unlike const, a constexpr value is evaluated during compilation when possible, which can improve performance.

Difference Between Variables and Constants

Example:

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1int age = 20;
2
3age = 21;

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1const int MAX_USERS = 100;

Common Beginner Mistakes

Using an Uninitialized Variable

Incorrect:

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1int age;
2
3cout << age;

Always initialize variables:

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1int age = 0;

Using Keywords as Variable Names

Incorrect:

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1int class = 10;

Correct:

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1int classNumber = 10;

Choosing Meaningless Names

Poor:

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1int x;
2int y;

Better:

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1int employeeAge;
2int totalSalary;

Best Practices

• Use descriptive variable names.
• Initialize variables when declaring them.
• Use const for values that never change.
• Follow camelCase naming conventions.
• Avoid single-letter variable names except in simple loops.
• Keep names concise but meaningful.

Practice Exercises

Exercise 1

Create variables for your name, age, and city, then print them.

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1#include <iostream>
2#include <string>
3
4using namespace std;
5
6int main()
7{
8    string name = "Ankit";
9    int age = 22;
10    string city = "Lucknow";
11
12    cout << "Name: " << name << endl;
13    cout << "Age: " << age << endl;
14    cout << "City: " << city;
15
16    return 0;
17}

Exercise 2

Declare two integer variables and print their values.

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1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    int a = 10;
8    int b = 20;
9
10    cout << a << " " << b;
11
12    return 0;
13}

Exercise 3

Declare a constant for the value of π and print it.

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1#include <iostream>
2
3using namespace std;
4
5int main()
6{
7    const double PI = 3.14159;
8
9    cout << PI;
10
11    return 0;
12}

Interview Questions

1. What is a variable in C++?
2. What is the difference between declaration and initialization?
3. What are the rules for naming variables?
4. What are literals in C++?
5. What is the difference between const and constexpr?
6. Why should variables be initialized?
7. Can a keyword be used as a variable name? Why or why not?

Summary

In this chapter, you learned:

• What variables are and why they are essential.
• How to declare and initialize variables.
• Rules and best practices for naming variables.
• What C++ keywords are.
• Different types of literals.
• How to use const and constexpr to create constants.
• Common mistakes to avoid when working with variables.

In the next chapter, you'll explore C++ Data Types, including int, float, double, char, bool, string, and auto, and learn how each type stores different kinds of data.