arith.cpp

#include <string>
#include <sstream>
#include <iostream>

class AddExp;
class MulExp;
class Number;

class ExpVisitor {
public:
  virtual void visit(const AddExp * const exp) = 0;
  virtual void visit(const MulExp * const exp) = 0;
  virtual void visit(const Number * const exp) = 0;

  virtual ~ExpVisitor() {};
};

class Exp { // base class for expressions
public:
  virtual ~Exp() {}
  virtual void accept(ExpVisitor *visitor) const = 0;
};

class AddExp : public Exp {
public:
  const Exp * const e1, * const e2;
  AddExp(const Exp * const e1, const Exp * const e2)
    : e1(e1), e2(e2) {}
  void accept(ExpVisitor *visitor) const { visitor->visit(this); }
};

class MulExp : public Exp {
public:
  const Exp * const e1, * const e2;
  MulExp(const Exp * const e1, const Exp * const e2)
    : e1(e1), e2(e2) {}
  void accept(ExpVisitor *visitor) const { visitor->visit(this); }
};

class Number : public Exp {
public:
  const float f;
  Number(const float f) : f(f) {}
  void accept(ExpVisitor *visitor) const { visitor->visit(this); }
};

class Run : public ExpVisitor {
public:
  float result;
  Run() : result(0) {}

  void visit(const AddExp * const exp) {
    Run v1;
    exp->e1->accept(&v1);

    Run v2;
    exp->e2->accept(&v2);

    result = v1.result + v2.result;
  }

  void visit(const MulExp * const exp) {
    Run v1;
    exp->e1->accept(&v1);

    Run v2;
    exp->e2->accept(&v2);

    result = v1.result * v2.result;
  }

  void visit(const Number * const exp) {
    result = exp->f;
  }
};

class StringOfExp : public ExpVisitor {
public:
  std::string result;
  StringOfExp() : result(std::string()) {}

  void visit(const AddExp * const exp) {
    StringOfExp v1;
    exp->e1->accept(&v1);

    StringOfExp v2;
    exp->e2->accept(&v2);

    result = "(" + v1.result + " + " +  v2.result + ")";
  }

  void visit(const MulExp * const exp) {
    StringOfExp v1;
    exp->e1->accept(&v1);

    StringOfExp v2;
    exp->e2->accept(&v2);

    result = "(" + v1.result + " * " +  v2.result + ")";
  }

  void visit(const Number * const exp) {
    std::ostringstream ss;
    ss << exp->f;
    result = ss.str();
  }
};

int main () {

  Exp *prog1 = new AddExp(
      new Number(10), new MulExp(
        new Number(20), new AddExp(
          new Number(30), new Number(40))));

  Run r;
  prog1->accept(&r);
  std::cout << "return value Run: " << r.result << std::endl;

  StringOfExp s;
  prog1->accept(&s);
  std::cout << "return value of StringOfExp: " << s.result << std::endl;

  return 0;
}