//person.h
#ifndef __PERSON_H__
#define __PERSON_H__
#include <string>

class person
{
public:
  //constructors and access functions
private:
  std::string first_;
  std::string last_;
  unsigned long id_;
  //...

  friend struct less_first;
  friend struct less_last;
  friend struct less_id;
  //...
};

struct less_first
{
  bool operator()(person const &lh, person const &rh) const
    {return lh.first_ < rh.first_;}
};

struct less_last
{
  bool operator()(person const &lh, person const &rh) const
    {return lh.last_ < rh.last_;}
};

struct less_id
{
  bool operator()(person const &lh, person const &rh) const
    {return lh.id_ < rh.id_;}
};

//...

#endif