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(int n)
{
  long long result = 1;
  for (int i = 1; i <= n; ++i) {
    result *= i;
  }
  return result;
}

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