Variable Declaration and Initialization

In C++, variable declaration and initialisation are fundamental concepts. Here’s a concise to explanation along with examples to illustrate them:

Variable Declaration

Declaring a variable means informing the compiler about the variable's name and type, but it doesn't allocate its name and type, nor does it allocate memory or assign a value.

Syntax:

type variable_name;

Example:

int age;
float height;
char grade;

Variable Initialization

Initializing a variable means assigning it an initial value at the time of declaration. This allocates memory and assigns the value.

Syntax:

type variable_name = value;

Example:

int age = 25;
float height = 5.9;
char grade = 'A';

Combining Declaration and Initialization

You can combine declaration and initialization in a single statement, which is a common practice.

Example:

int age = 25;
float height = 5.9;
char grade = 'A';

Multiple Variables

You can declare and initialize multiple variables of the same type in one line.

Example:

int x = 10, y = 20, z = 30;

Constant Variables

You can declare constant variables that cannot be changed after initialization using the const keyword.

Example:

const int MAX_AGE = 100;
const float PI = 3.14159;

Different Types of Initialization

C++ offers various ways to initialize variables, including:

Copy Initialization:

int age = 25;

Direct Initialization:

int age(25);

Uniform Initialization (C++11 and later):

int age{25};

Example Code

Here’s a complete example demonstrating different types of variable declarations and initializations:

#include <iostream>
int main() {
   // Declaration
   int age;
   float height;
   char grade;
   // Initialization
   age = 25;
   height = 5.9;
   grade = 'A';
   // Declaration and Initialization
   int weight = 70;
   float bmi = 22.5;
   char initial = 'J';
   // Constant variables
   const int MAX_AGE = 100;
   const float PI = 3.14159;
   // Direct Initialization
   int year(2024);
   // Uniform Initialization
   int month{8};
   std::cout << "Age: " << age << std::endl;
   std::cout << "Height: " << height << std::endl;
   std::cout << "Grade: " << grade << std::endl;
   std::cout << "Weight: " << weight << std::endl;
   std::cout << "BMI: " << bmi << std::endl;
   std::cout << "Initial: " << initial << std::endl;
   std::cout << "Max Age: " << MAX_AGE << std::endl;
   std::cout << "PI: " << PI << std::endl;
   std::cout << "Year: " << year << std::endl;
   std::cout << "Month: " << month << std::endl;
   return 0;
}

This example covers variable declaration, initialization, and the use of constants, demonstrating different methods of initializing variables in C++.