In C++, const is a keyword used to define constant variables and functions. It ensures that a variable or function does not alter its value or behavior during the program’s execution. Using const enhances the reliability of the code by preventing unintended changes. The const Data Members are used for the purpose that if we donot want to change the value of any variable and want to make it constant.

Code Example: const Data Members and Functions in C++

#include <iostream>
using namespace std;

class Circle {
private:
    const double pi = 3.14; // Constant member for the value of pi
    double radius;

public:
    // Constructor to initialize the radius
    Circle(double r) : radius(r) {}

    // Const member function to calculate the area
    double calculateArea() const {
        return pi * radius * radius;
    }

    // Const member function to display the radius
    void displayRadius() const {
        cout << "Radius: " << radius << endl;
    }
};

int main() {
    Circle circle(5.0); // Create an object with radius 5.0
    circle.displayRadius(); // Display the radius
    cout << "Area of the circle: " << circle.calculateArea() << endl; // Display the area
    return 0;
}

Explanation of the Code:

1. Class Definition (Circle):

a) const Data Member (const double pi):

• Definition:
  • pi is declared as const, meaning its value cannot be modified after initialization.
  • It is initialized with 3.14 and remains constant throughout the program.
• Purpose:
  • Used to represent the mathematical constant π, ensuring its value remains fixed for accurate area calculations.

b) Non-Constant Data Member (double radius):

• A regular variable that stores the radius of the circle. This can be modified as needed through object initialization or other operations.

c) Constructor (Circle(double r)):

• Purpose:
  • Initializes the radius attribute using the value provided during object creation.
• Syntax:
  • Circle(double r) : radius(r) uses an initializer list to set the radius.

d) const Member Functions:

1. double calculateArea() const:
   • Purpose:
     • Calculates the area of the circle using the formula Area=π×radius2\text{Area} = \pi \times \text{radius}^2Area=π×radius2.
   • Why const?
     • Declared as const to ensure it does not modify any data members of the class.
     • This function guarantees that it only reads the values of pi and radius and performs calculations without altering them.
2. void displayRadius() const:
   • Purpose:
     • Displays the radius of the circle.
   • Why const?
     • Ensures the function does not modify the radius or any other data members.

Key Concept of const Member Functions:

• Functions marked with const can only call other const functions and access const data members or regular data members in a read-only manner.

2. Main Function:

int main() {
    Circle circle(5.0); // Create an object with radius 5.0
    circle.displayRadius(); // Display the radius
    cout << "Area of the circle: " << circle.calculateArea() << endl; // Display the area
    return 0;
}

Explanation:

1. Object Creation (Circle circle(5.0);):
   • An object circle of class Circle is created with a radius of 5.0.
   • The constructor initializes the radius member to 5.0.
2. Calling displayRadius() (circle.displayRadius();):
   • Outputs the radius of the circle:makefileCopy codeRadius: 5
3. Calling calculateArea() (circle.calculateArea();):
   • Computes and outputs the area of the circle using the formula π×radius2\pi \times \text{radius}^2π×radius2:arduinoCopy codeArea of the circle: 78.5

Output of the Program:

Radius: 5
Area of the circle: 78.5

Key Concepts of const in C++:

1. const Data Members:

• A const data member is a variable declared with the const keyword.
• It must be initialized at the time of declaration (if defined inside the class) or in the constructor using an initializer list.
• Advantages:
  • Prevents accidental modification of critical data.
  • Useful for constants like mathematical values (e.g., pi) or configuration settings.

2. const Member Functions:

• Declared with the const keyword at the end of the function signature:cppCopy codedouble calculateArea() const;
• These functions:
  • Cannot modify any class members.
  • Can only call other const member functions.
• Advantages:
  • Helps in writing safe code by preventing unintentional modifications.
  • Makes the class usable with constant objects.

Advantages of Using const:

1. Code Safety:
   • Protects against unintended changes to critical variables or functions.
2. Improved Readability:
   • Clearly indicates which parts of the code are immutable.
3. Better Compatibility:
   • Allows the class to be used with const objects or references.

Conclusion:

Using const data members and functions in C++ is an essential practice for writing robust and secure code. In the provided example, the const member pi ensures the value of π remains unchanged, while the const member functions guarantee that operations like calculateArea() and displayRadius() do not modify the circle’s data. By leveraging const, developers can create classes that are safer, easier to debug, and compatible with const objects.