Constants in C++

Constants in C++ are values that remain unchanged throughout the execution of a program. They help improve code reliability, readability, and maintainability by preventing unintended modifications.

• Can be defined using const, constexpr, or #define.
• Used to represent fixed values such as limits, configuration settings, and mathematical constants.
• Help make code easier to understand and maintain.

#include <iostream>
using namespace std;

int main() {

    // Declaring and defining a constant variable
    const int c = 24;

  	cout << c;
    return 0;
}

24

Explanation: The variable c is declared as a constant using the const keyword. Any attempt to modify its value later in the program will result in a compilation error.

Types of Constants in C++

In C++, constants can be classified based on how they are created. They can be defined using the following three ways:

1. Constants Using const

The const keyword is used to declare a variable whose value cannot be changed after initialization.

Syntax

const DATATYPE variable_name =value;

#include <iostream>
using namespace std;

int main() {
    int var = 10;

    // Declaring a constant variable
    const int c = 24;

    // Trying to change the value constant c
    c = 0;

    cout << c;
    return 0;
}

Output

./Solution.cpp: In function 'int main()':
./Solution.cpp:19:10: error: assignment of read-only variable 'c'
     cons = 0;

Explanation: The variable c is initialized with the value 24. Since it is declared as const, attempting to modify it later results in a compilation error.

2. Constants Using constexpr

The constexpr keyword is used to declare compile-time constants in C++. Unlike const, a constexpr variable must be initialized with a value that is known at compile time.

Syntax

constexpr DATATYPE variable_name = value ;

#include <iostream>
using namespace std;

int main() {
  
    // Defining constant
    int constexpr hoursIn_day = 24;

    // Printing value
    cout << hoursIn_day;
  	return 0;
}

24

Explanation hoursInDay is evaluated at compile time, allowing the compiler to perform additional optimizations.

3. Constants Using #define

The #define directive creates macro constants by replacing names with values during preprocessing. It is less preferred because it does not provide type safety.

Syntax

#define MACRO_NAME replacement_value

#include <iostream>
using namespace std;

// Using #define to create a macro
#define Side 5

int main()
{
    // Using constant
    double area = Side * Side;

    cout << area;
    return 0;
}

25

Explanation: The preprocessor replaces every occurrence of Side with 5 before compilation begins.

Note: Macro constants are generally less preferred than const or constexpr because they do not provide type safety.

Real-World Examples of Constants

ATM Daily Withdrawal Limit: In banking software, a customer may have a fixed limit of ₹20,000 per day.

const int dailyLimit = 20000;

Max Login Attempts: Apps like WhatsApp or banking apps usually allow only 3 login attempts.

const int maxLoginAttempts = 3;

Value of Pi (π): The value of π is always the same in mathematical formulas.

const float pi = 3.14159;

Advantages of Constants

Using constants provides several benefits:

• Prevents accidental modification of values.
• Improves code readability and maintainability.
• Makes programs easier to update by avoiding hard-coded values.
• Enables compiler optimizations in some cases.

Limitations of Constants

Although useful, constants have some restrictions:

• A const variable cannot be modified after initialization.
• Constants must usually be initialized when declared.
• Incorrect use of constants with pointers can be confusing for beginners.

Note: Constants and literals are different concepts in C++. For a detailed comparison, see Constants vs Literals in C++