consteval and constinit

#include <iostream>
// consteval: MUST be evaluated at compile time
consteval int square(int n) {
    return n * n;
}
// constexpr: CAN be evaluated at compile time
constexpr int cube(int n) {
    return n * n * n;
}
// constinit: must be initialized at compile time
// but can be modified at runtime
constinit int global_value = square(5);  // OK: compile-time init
// Useful for avoiding static initialization order fiasco
constinit const char* app_name = "MyApp";
// consteval for compile-time only computation
consteval auto create_lookup_table() {
    std::array<int, 10> table{};
    for (int i = 0; i < 10; ++i) {
        table[i] = i * i;
    }
    return table;
}
constexpr auto squares_table = create_lookup_table();
int main() {
    constexpr int a = square(5);   // OK: compile-time
    // int x = 5;
    // int b = square(x);          // ERROR: x not constexpr
    
    int y = 3;
    int c = cube(y);               // OK: runtime evaluation
    constexpr int d = cube(4);     // OK: compile-time
    
    std::cout << "square(5) = " << a << std::endl;  // 25
    std::cout << "cube(3) = " << c << std::endl;    // 27
    
    // constinit variable can be modified
    global_value = 100;
    std::cout << "global = " << global_value << std::endl;
    
    // Use lookup table
    std::cout << "squares[7] = " << squares_table[7] << std::endl;  // 49
    
    return 0;
}