What are Variables and How to Use Them in C++

A variable is a reserved space in memory with a symbolic name (identifier), where we can store a value of a specific type.

Variables allow us to store temporary data that we can use later throughout our program.

Variable Declaration

Declaring a variable in C++ means specifying its type and giving it a name. The basic syntax for declaring a variable is:

type variableName;

For example, to declare an integer variable named age:

int age;

Value Assignment

Assigning a value to a variable means giving it an initial value or modifying its existing value. The syntax for assigning a value to a variable is:

variableName = value;

For example, to assign the value 25 to the variable age:

age = 25;

It is also possible to declare and assign a variable in a single line:

int age = 25;

Default Values

C++ does not automatically initialize variables with default values. Uninitialized local variables contain garbage binary (a random value) until they are explicitly assigned a value.

Value Types

For value types (int, float, double, char, bool, etc.), it is advisable to initialize them at the time of declaration:

int age = 0;
float height = 0.0f;
double weight = 0.0;
char initial = '\0';
bool isStudent = false;

Reference Types

For reference types (like pointers), the default value is nullptr, which indicates that the pointer does not reference any location in memory.

int* pointer = nullptr;
std::string* string = nullptr;

Volatile Variables

volatile variables indicate to the compiler that the value of the variable may change at any time, outside the control of the program.

volatile int interrupt;

Practical Examples

Let’s look at some simple examples of using variables in C++. These examples show how to declare and use variables to perform calculations and manage data.

Managing Student Data

In this example, we show how to manage basic data of a student, such as their name and age. This example illustrates how to declare and assign values to variables of different data types.

#include <iostream>
#include <string>

int main() {
    // Declare variables
    std::string name;  // for the student's name
    int age;           // for the student's age

    // Assign values to the variables
    name = "Ana"; 
    age = 21;

    // Print the student's data to the console
    std::cout << name << " is " << age << " years old" << std::endl;

    return 0;
}

Circle Area Calculation

In this example, we calculate the area of a circle using a basic mathematical formula. A constant is declared for the value of PI and a variable for the radius of the circle.

#include <iostream>

int main() {
    // Declare a constant for the value of PI
    const double PI = 3.14159;

    // Declare a variable for the radius of the circle
    double radius = 5.0;

    // Calculate the area of the circle using the formula: area = PI * radius^2
    double area = PI * radius * radius;

    // Print the result to the console
    std::cout << "The area of the circle is: " << area << std::endl;

    return 0;
}