How do I write constexpr functions?

In C++, a `constexpr` function is a function that is evaluated at compile time rather than at runtime. This can lead to performance improvements and allows for more complex computations to be done during compilation. To define a `constexpr` function, you simply use the `constexpr` keyword before the return type of the function. Below is an example of a simple `constexpr` function that computes the factorial of a number.


        constexpr int factorial(int n) {
            return (n <= 1) ? 1 : n * factorial(n - 1);
        }

        int main() {
            constexpr int result = factorial(5);
            // result will be 120 at compile time
        }