C++ Constant

In C++, a constant is a variable whose value cannot be changed after it is initialized. Once a constant is set, you cannot modify its value throughout the program. Constants are useful when you want to ensure that a certain value stays the same and doesn’t get accidentally modified.

A constant is an explicit number or character (such as 1, 0.5, or ‘c’) that doesn’t change.

In C++, you can define constants using the const keyword or #define preprocessor directive.

Using the const keyword:


#include <iostream>
using namespace std;

int main() {
    const int MAX_SPEED = 180;  // constant integer
    cout << "Car's maximum speed is: " << MAX_SPEED << endl;

    // Uncommenting the next line will cause a compile-time error:
    // MAX_SPEED = 150;  // Error! Cannot modify a constant value

    return 0;
}

Explanation:

  1. MAX_SPEED is a constant that holds the value 180.
  2. The const keyword ensures that the value of MAX_SPEED cannot be changed after initialization.

Output:

Car's maximum speed is: 180

Using #define Preprocessor Directive:


#include <iostream>
using namespace std;

#define MAX_SPEED 180  // Defining a constant using #define

int main() {
    cout << "Car's maximum speed is: " << MAX_SPEED << endl;

    // You can't modify MAX_SPEED, as it's a constant
    // Uncommenting the next line will cause a compile-time error:
    // MAX_SPEED = 150;  // Error!

    return 0;
}

Explanation:

  1. define MAX_SPEED 180 creates a constant MAX_SPEED with the value 180.
  2. define is a preprocessor directive and is replaced at compile time, so it works a bit differently than const.

Output:

Car's maximum speed is: 180

Why Use Constants?

Safety: Constants prevent accidental changes to important values.

Clarity: By using meaningful names for constants, you make your code more readable and self-documenting.

Maintainability: If you need to change the value of a constant, you only have to change it in one place, rather than hunting for all instances where it’s used.

C++ Constant – Interview Questions

Q 1: What is a constant?
Ans: A constant is a value that cannot be changed.
Q 2: How do you define a constant?
Ans: Using the const keyword.
Q 3: Can constants be modified?
Ans: No.
Q 4: Why use constants?
Ans: For code safety and readability.
Q 5: Example of constant declaration?
Ans: const int MAX = 10;

C++ Constant – Objective Questions (MCQs)

Q1. Which keyword is used to define a constant variable in C++?





Q2. What will happen if you try to change the value of a variable declared as const in C++?





Q3. Which of the following correctly defines a constant integer in C++?





Q4. Which preprocessor directive is used to define a symbolic constant in C++?





Q5. What is the output of the following code?

const int x = 5;
cout << x;





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