Variables in C++

A variable is a named location used to store data. For example:

int age = 20;

Here, age is a variable that stores the integer value 20.

Variables allow a program to store information and use that information later.

• Store numbers
• Store characters
• Store text
• Store true or false values
• Store calculated results
• Store user input

Variable declaration tells the compiler the variable's data type and name.

int age;

Here:

• int is the data type.
• age is the variable name.

Initialization means giving a variable an initial value when it is created.

int age = 20;

The variable age is declared and initialized with the value 20.

A variable can be declared first and assigned a value later.

int age;

age = 20;

The first statement declares the variable and the second statement assigns a value to it.

C++ provides several fundamental data types that can be used for variables.

• int – integer values
• float – single-precision floating-point values
• double – double-precision floating-point values
• char – a character
• bool – true or false

An int variable is used to store integer values.

int age = 20;
int marks = 85;

Integer values do not contain a fractional part.

A float variable can store a floating-point value.

float price = 99.50f;

The f suffix indicates a float literal.

The double type is commonly used when more precision is needed than a typical float.

double salary = 25000.75;

A char variable stores a character.

char grade = 'A';

Character literals are commonly written using single quotation marks.

A bool variable represents a Boolean value: true or false.

bool passed = true;

The C++ standard library provides std::string for storing text. It is available through the <string> header.

#include <string>

std::string name = "Rahul";

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

int age = 20, marks = 80;

However, declaring related variables separately can sometimes make code easier to read.

A variable's value can be changed by assigning a new value to it.

int age = 20;

age = 25;

After the second statement, age contains 25.

Variables can be used in arithmetic expressions.

int a = 10;
int b = 20;

int sum = a + b;

std::cout << sum;

The variable sum stores the result of the addition.

The value of a variable can be displayed using std::cout.

#include <iostream>

int main() {

    int age = 20;

    std::cout << age;

    return 0;
}

#include <iostream>

int main() {

    int age = 20;

    std::cout << "Age = " << age;

    return 0;
}

Multiple values can be sent to std::cout using the << operator.

The std::cin object can be used to store user input in a variable.

#include <iostream>

int main() {

    int age;

    std::cout << "Enter your age: ";
    std::cin >> age;

    std::cout << "Age = " << age;

    return 0;
}

The const keyword can be used when a variable should not be modified after initialization.

const double PI = 3.14159;

Attempting to modify a const object after initialization is not allowed.

Variable names should be meaningful and follow C++ identifier rules.

int studentAge = 20;
double courseFee = 5000;

Meaningful names make programs easier to understand.

Examples of valid variable names:

age
studentAge
student_age
marks1
totalMarks

These names follow the basic identifier rules.

Examples of invalid variable names:

2age
student name
my-name
class

A variable name cannot begin with a digit, contain spaces or hyphens, or be a reserved C++ keyword.

C++ is case-sensitive, so these are different variables:

int age = 20;
int Age = 30;
int AGE = 40;

It is generally better to use a consistent naming style in a project.

A variable declared inside a function or block is generally a local variable. Its scope is limited to that function or block.

int main() {

    int age = 20;

    std::cout << age;

    return 0;
}

A variable declared outside functions is a global variable. Its scope and lifetime differ from those of local variables.

#include <iostream>

int count = 10;

int main() {

    std::cout << count;

    return 0;
}

Global variables should be used carefully because they can be accessed from multiple parts of a program depending on their linkage and scope.

Scope describes the region of a program where a name can be used.

int main() {

    int age = 20;

    if (age > 18) {

        int bonus = 5;

        std::cout << bonus;
    }

    return 0;
}

The variable bonus is declared inside the if block and its scope is limited to that block.

C++ supports several initialization forms.

int a = 10;

int b(20);

int c{30};

Brace initialization is a modern C++ style and can help prevent certain narrowing conversions.

Modern C++ provides the auto keyword for type deduction. The compiler determines the variable's type from its initializer.

auto age = 20;
auto price = 99.50;

In the first example, age is deduced as an integer type, while the second is deduced as a floating-point type.

#include <iostream>
#include <string>

int main() {

    std::string name = "Rahul";
    int age = 20;
    double marks = 85.5;
    char grade = 'A';
    bool passed = true;

    std::cout << "Name: " << name << std::endl;
    std::cout << "Age: " << age << std::endl;
    std::cout << "Marks: " << marks << std::endl;
    std::cout << "Grade: " << grade << std::endl;
    std::cout << "Passed: " << passed << std::endl;

    return 0;
}

This program demonstrates variables of several common C++ types.

Variables are fundamental to C++ programming. They allow programs to store, process, modify, and display data.