两道constexpr面试题，拿下就稳了

#include <iostream>

struct Foo {
  static constexpr int x = 42;
};

int f(const int& r) {
  std::cout << r << std::endl;
  return 0;
}

int main() {
  std::cout << Foo::x << std::endl;
  int array[Foo::x] = {0};
  const int& x = Foo::x;
  std::cout << x << std::endl;
  int a;
  std::cin >> a;
  int n = a ? Foo::x : f(Foo::x);
  std::cout << n << ", " << array[0] << std::endl;
}

struct S {
  static constexpr int x = 42;
};

constexpr int S::x;