/*
 * example based on R. Grimm, C++ Core Guidelines Explained, p. 443.
 */

#include <iostream>
#include <gsl/gsl>

namespace
{
   const int lineskip_characters = 255;
}

int area(int height)
{
   Expects(height > 0);  // *** here we check a precondition ***
   
   std::cout << "Input a natural number: width = ";
   int width = 0;
   std::cin >> width;
   std::cin.ignore(lineskip_characters, '\n');  // ignore rest of the input
   
   int retv = height*width;
   Ensures(retv > 0);  // *** here we check a postcondition ***
   return retv;
}

int main()
{
   std::cout << "Input a natural number: height = ";
   int h = 0;
   std::cin >> h;
   std::cin.ignore(lineskip_characters, '\n');  // ignore rest of the input
   Ensures(h > 0);  // *** here we check a run-time condition ***

   int a = area(h);
   std::cout << "Product: area = " << a << "\n";
}