Subject: code for posting -start point for hw6


// Expression parsing  by Ira Pohl  
// Extend this example as indicated in HW6
#include <iostream.h>

//Abstract Base Class + Reference Collected Type
class Token {   //allowed are integers and binary operators
protected:
   friend class Expr;
   friend ostream& operator<<(ostream&, const Expr&);
   Token(){ use = 1; }
   virtual void print(ostream&) = 0;
   virtual ~Token() {}     //note virtual destructor
   virtual int eval() = 0;
private:
   int use;    //reference count
};

class Expr {
public:
   friend class Token;
   friend ostream& operator<<(ostream&, const Expr&);
   Expr(int);                //constant
   Expr(char*, Expr, Expr);  //binary operator
   Expr(const Expr& t) { p = t.p; ++p -> use;}
   ~Expr() {if (--p ->use == 0) delete p; }
   void operator=(const Expr& t);
   int eval() {return p->eval();}
private:
   Token* p;  //polymorphic hierarchy
};

void Expr:: operator=(const Expr& t)
{
   ++t.p -> use;
   if ( --p -> use == 0) 
      delete p;
   p = t.p;
}

ostream& operator<<(ostream& o, const Expr& t)
{
   t.p -> print(o);
   return (o);
}



class IntToken: public Token {
public:
   friend class Expr;
   int eval(){ return n;}
private:
   int n;
   void print(ostream& o) { o << n ;}
   IntToken(int k): n (k) {}
};



class BinaryToken: public Token {
public:
   friend class Expr;
   int eval();
private:
   char* op;
   Expr left;
   Expr right;
   BinaryToken(char* a, Expr b, Expr c): op(a), left(b), right(c){}
   void print (ostream& o) {o << "(" << left << op << right << ")";}
};

int BinaryToken::eval()
{
   switch (op[0]){
   case '-': return (left.eval() - right.eval());
   case '+': return (left.eval() + right.eval());
   case '*': return (left.eval() * right.eval());
   default: cerr << "no operand\n" << endl;
      return 0;
   }
}


Expr::Expr(int n)
{
   p = new IntToken(n);
}



Expr::Expr(char* op, Expr left, Expr right)
{
   p = new BinaryToken(op, left, right);
}

int main()
{
   
   Expr t1 = Expr("*", Expr("-", 5, 8), Expr("+", 6, 4));
   Expr t2 = Expr("+", Expr("-", 4, 1), Expr("+", t1, 2));
   cout << "t1 = " << t1 << " ;  t2 = " << t2 << endl;
   cout << "t1:" << t1.eval() << "  t2:" << t2.eval() << endl;
}