What are the applications of constexpr in C++?

In C++, the constexpr keyword is used to declare constant expressions that can be evaluated at compile time. Here are some scenarios where constexpr is useful:

1. Calculating constant values: using constexpr allows for calculating constant values at compile time instead of runtime, which improves program performance. For example, constexpr can be used to calculate the factorial of a number or Fibonacci sequence.

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

constexpr int fib(int n) {
    return (n <= 1) ? n : (fib(n - 1) + fib(n - 2));
}

int main() {
    constexpr int fact5 = factorial(5);
    constexpr int fib10 = fib(10);
    return 0;
}

1. Before C++11, the size of an array had to be a constant expression. By using constexpr, the size of an array can be calculated at compile time.

constexpr int size = 10;
int array[size];

1. After C++11, constexpr can be used as a template parameter to make templates more versatile. For example, constexpr can be used as a template parameter to specify the size of an array.

template <int Size>
struct Array {
    int data[Size];
};

int main() {
    Array<10> arr;
    return 0;
}

1. For enumerations: using constexpr allows you to declare constants in enumeration types, providing validation at compile time and offering improved type safety.

enum class Color {
    RED = 0,
    GREEN = 1,
    BLUE = 2,
};

constexpr Color mix(Color c1, Color c2) {
    return static_cast<Color>(static_cast<int>(c1) | static_cast<int>(c2));
}

int main() {
    constexpr Color c = mix(Color::RED, Color::BLUE);
    return 0;
}

1. For constant functions: By using the constexpr keyword to decorate the function, it ensures that the function is evaluated at compile time and can only accept constant expressions as parameters.

constexpr int square(int n) {
    return n * n;
}

constexpr int result = square(5);

These are just some of the applications of constexpr, which can be used in many other situations to improve performance or enhance the readability and maintainability of the code.