#include <string>
#include <iostream>

using namespace std;

class StringDoer {
public:
  virtual void f(string) = 0;
};

class StringSet { // no superclass unlike in homework
public:
  StringSet();
  virtual ~StringSet();
  bool member(string);
  void add(string);
  void foreach(StringDoer*);
private:
  int num_elts;
  int array_size;
  string * array;
};

void StringSet::foreach(StringDoer * d) {
  for(int i=0; i < num_elts; ++i)
    d->f(array[i]);
}

// works great, different clients can provide different subclasses
// of StringDoer with fields (and extra methods) of any type they want

// Simple example:
class StringLengthSummer: public StringDoer {
public:
  StringLengthSummer() : sofar(0) {}
  virtual ~StringLengthSummer() {}
  virtual void f(string s) { sofar += s.length(); }
  virtual int getSum() { return sofar; }
private:
  int sofar;
};

int length_summer(StringSet * set) {
  StringLengthSummer * d = new StringLengthSummer();
  set->foreach(d); // upcast!
  int ans = d->getSum();
  delete d;
  return ans;
}