Essential C++ : Calculating Factorials : https://www.programming-books.io/essential/cpp/calculating-factorials-317f96c51a284c9ab5e0bf64625f5d70 (made with https://codeeval.dev)

main.cc

#include <iostream>

constexpr long long factorial(long long n)
{
  if (n == 0)
    return 1;
  else
    return n * factorial(n - 1);
}

int main()
{
  char test[factorial(3)];
  std::cout << factorial(7) << '\n';
}