C++ Constants

In C++, constants are values that remain unchangeable throughout the program. If a value should stay the same such as fixed rates or system limits—it should be declared as a constant using the const keyword.

Declaring a Constant Variable

To prevent a variable’s value from being modified, use const before the type:

Example

#include <iostream>
using namespace std;

int main() {
    const int maxSpeed = 300;   // Declaring a constant variable

    // Attempting to modify the constant value
    maxSpeed = 200;  // This line will cause a compilation error

    cout << "Max Speed: " << maxSpeed << " km/h" << endl;

    return 0;
}

#include <iostream>

using namespace std;

​

int main() {

    const int maxSpeed = 300;   // Declaring a constant variable

​

    // Attempting to modify the constant value

    maxSpeed = 200;  // This line will cause a compilation error

​

    cout << "Max Speed: " << maxSpeed << " km/h" << endl;

​

    return 0;

}

Explanation:

• maxSpeed is set as a constant, meaning it cannot be changed later.
• Trying to assign a new value causes an error.

When to Use Constants

If a value should never change, always declare it as a constant to avoid accidental modifications.

Example

const int hoursPerDay = 24;

const int hoursPerDay = 24;

​

This ensures the value always remains 24 throughout the program.

Important Notes About Constants

• A constant must be assigned a value at declaration:

const int rpmLimit = 8000;

const int rpmLimit = 8000;

• This will not work (causes an error):

const int rpmLimit; // No value assigned rpmLimit = 8000;
rpmLimit = 8000; // error: cannot assign to a constant

const int rpmLimit; // No value assigned rpmLimit = 8000;

rpmLimit = 8000; // error: cannot assign to a constant