constexpr

Code

#include <iostream>
#include <array>

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

// constexpr variable
constexpr int max_size = 256;

// constexpr class (C++20)
struct Point {
    int x, y;
    constexpr Point(int x, int y) : x(x), y(y) {}
    constexpr int sum() const { return x + y; }
};

// consteval: must execute at compile time (C++20)
consteval int square(int n) {
    return n * n;
}

// Compile-time Fibonacci
constexpr int fib(int n) {
    if (n <= 1) return n;
    int a = 0, b = 1;
    for (int i = 2; i <= n; ++i) {
        int tmp = a + b;
        a = b;
        b = tmp;
    }
    return b;
}

int main() {
    // Compile-time computation
    constexpr int f5 = factorial(5);
    std::cout << "5! = " << f5 << "\n";

    // Used for array size
    std::array<int, factorial(3)> arr;
    std::cout << "array size: " << arr.size() << "\n";

    // constexpr object
    constexpr Point p(3, 4);
    static_assert(p.sum() == 7);

    // Compile-time vs runtime
    int n = 5;
    int runtime_f = factorial(n); // Runtime
    constexpr int compile_f = factorial(5); // Compile-time

    std::cout << "fib(10) = " << fib(10) << "\n";
    std::cout << "square(7) = " << square(7) << "\n";

    return 0;
}

Explanation

constexpr allows compile-time computation; consteval forces compile-time execution, improving runtime performance.

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