Make the reverse-polish calculator available for use in any program that wants to use it.
This should be based on a stack of doubles, for which you may use a global variable.

You should provide at least four functions:

void start_calc();
   makes sure the stack is empty.

void deal_with(double val);
   push the value onto the stack

void deal_with(string op);
   for operators: peform the operation indicated by the string.
   You should cover at least "+", "-", "*", "/" and a few more to
   make it useful.

double answer();
   return the current answer from the stack();

For example, to print out the value of (2+3)*(4+5):
  start_calc();
  deal_with(2);
  deal_with(3);
  deal_with("+");
  deal_with(4);
  deal_with(5);
  deal_with("+");
  deal_with("*");
  cout << answer();  

ERROR-CHECKING: Make sure your program never crashes:
  detect and avoid attempts to divide by zero
  never pop from an empty stack
  etc