const

Learn to declare immutable variables with the const keyword.

const

The const keyword makes a variable immutable. After declaring const int maxUsers = 100;, writing maxUsers = 200; gives a compile error.

const tells the compiler and the reader that this value will not change. This improves reliability and helps the compiler optimize.

Using const for constants is a good habit: const double pi = 3.14159;. Add const wherever you do not want a value to change.

Using const

A const variable must be given a value when declared; it cannot be assigned later. const int x; alone is invalid; const int x = 5; is correct.

The value of a const variable can be determined at runtime. You can hold a function result as a constant: const int y = readValue(); — the value is computed when the program runs but never changes afterward.

If the value is known at compile time, constexpr may be more appropriate; you will see it in the next lesson.

Exercise Define constants and print

Inside main, declare two const variables: maxUsers with value 100 (int) and taxRate with value 0.18 (double). Print both with std::cout using std::endl.

Show example solution

#include <iostream>

int main() {
    // Define constants with const
    const int maxUsers = 100;
    const double taxRate = 0.18;
    // Print with std::cout
    std::cout << maxUsers << std::endl;
    std::cout << taxRate << std::endl;
    return 0;
}

Test yourself

Defining a constant

____ double pi = 3.14159;

Show answer

const double pi = 3.14159;

What happens if you reassign a const variable?
```
const int x = 5;
x = 10;
```
- The value changes
- The program crashes
- A warning is given but it changes
- A compile error is raised
Show answer

A compile error is raised — Reassigning a const variable is a compile error.
Match const concepts

const

Immutable
reassignment

Compile error
declaration time

Value given
optimization

Helps compiler

Next lesson: constexpr

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