// Code derived from Stroustrup's PPP2 book
// § 6.5.2.3 Expressions: third time lucky
//  -and beginning on p 200

/*------------------------------------------------------------------------------

  Note: One of the goals behind the textbook's teaching for this section is to
  use an incremental approach for the student's understanding of all the various
  problems to be solved when creating a working calculator program.

   -As such, some of the examples from Ch 6 are non-working if left as-is.
   -This example is one of those.
   -The actual definitions of the function declarations:
        get_token(),  term()
    are being left till later on in the examples (so the build's linker step
    will fail therefore). For now, these are just placeholders for the ideas.

  A working example is already created for the classroom so you can look ahead.

------------------------------------------------------------------------------*/

class Token {
 public:
  Token(char ch) : kind{ch} {}

  Token(char ch, double val) : kind{ch}, value{val} {}

  char   kind  = '0';
  double value = 0.0;
};

Token  get_token();  // read a token from cin - (TBD!)
double term();       // read and evaluate a Term - (TBD!)

double expression()
{
  double left = term();  // read and evaluate a Term

  Token t = get_token();  // get the next token

  while (true) {
    switch (t.kind) {
      case '+':
        left += term();  // evaluate Term and add
        break;
      case '-':
        left -= term();  // evaluate Term and subtract
        break;
      default: return left;  // finally: no more + or -: return the answer
    }

    t = get_token();
  }
}

int main() { expression(); }