/in/foo.cc: In function 'int main()':
/in/foo.cc:13:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.size(); i++)
                  ~~^~~~~~~~~~
/in/foo.cc:26:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int j = i; j < s.size(); j++)
                   ~~^~~~~~~~~~
/in/foo.cc:46:12: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   while (c = '+' && !oper2.empty())
          ~~^~~~~~~~~~~~~~~~~~~~~~~