#include <iostream>
#include <string>

using namespace std;

struct input
{ string line;
  int pos, len;
          // function inside struct
          //          no return type
          //          same name as struct
          // is an automatic constructor
  input()
  { line = "";
    pos = 0;
    len = 0; }

  void fill(string s)
  { line = s;
    pos = 0;
    len = s.length(); }

  void print()
  { cout << "line is '" << line << "'\n";
    cout << "pos is " << pos << "\n";
    cout << "len is " << len << "\n"; }

  char next()
  { if (pos >= len)
      return '\n';
    char result = line[pos];
    pos += 1;
    return result; }

  void un_next()
  { if (pos <= 0)
      return;
    pos -= 1; }
 };

string read_atom(input & x)
{ string s = "";
  char c1 = x.next();
  if (c1 < 'A' || c1 > 'Z')
  { cout << "Capital expected at beginning of atom\n";
    return "XXXXXX"; }
  s += c1;
  char c2 = x.next();
  if (c2 >= 'a' && c2 <= 'z')
    return s + c2;
  x.un_next();
  return s; } 

int main()
{ input I;
  string s;
  cout << "Enter a line: ";
  getline(cin, s);
  I.fill(s);  // internal function of input object: Method
  for (int x = 0; x < 6; x += 1)
  { string at = read_atom(I);
    cout << "Atom = '" << at << "'\n";
    I.print(); }
}
/*
  while (true)
  { I.print();
    char c = I.next();
    cout << "Got the character '" << c << "'\n";
    if (c == '\n')
      break; } }
*/