Зачем для указателя на объект класса создается отдельный класс

class Foo
{
public:
...
private:
...
};
 
class PFoo
{
private:
Foo *foo;
public:
...
};

#include <iostream>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
 
class Book {
  char **authors_;
  size_t authors_count_;
  char *title_;
  int year_;
public:
  Book();
  ~Book();
  Book(const Book& rhl);
  Book& operator=(const Book& rhl);
 
  void addAuthor(const char* name);
  void setTitle(const char* title);
  void setYear(const int year);
  char* toString() const;
};
 
Book::Book()
  : authors_(NULL), 
    authors_count_(0),
    title_(NULL),
    year_(0)
{
}
 
Book::~Book() {
  if (authors_count_ > 0) {
    for(size_t i=0; i<authors_count_; ++i) {
      delete [] authors_[i];
    }
    delete [] authors_;
  }
  if (title_) {
    delete [] title_;
  }
}
 
Book::Book(const Book& rhl)
  : authors_(NULL), 
    authors_count_(rhl.authors_count_),
    title_(NULL),
    year_(rhl.year_)
{
    if (authors_count_>0) {
      authors_ = new char* [authors_count_];
      for(size_t i=0; i<authors_count_; ++i) {
        authors_[i] = new char [strlen(rhl.authors_[i])+1];
        strcpy(authors_[i], rhl.authors_[i]);
      }
    }
    if (rhl.title_) {
      title_ = new char [strlen(rhl.title_)+1];
      strcpy(title_, rhl.title_);
    }
}
 
Book& Book::operator=(const Book& rhl) {
  if (this == &rhl) return *this;
  
  if (authors_count_ > 0) {
    for(size_t i=0; i<authors_count_; ++i) {
      delete [] authors_[i];
    }
    delete [] authors_;
    authors_ = NULL;
  }
  if (title_) {
    delete [] title_;
  }
  authors_count_ = rhl.authors_count_;
  if (authors_count_>0) {
    authors_ = new char* [authors_count_];
    for(size_t i=0; i<authors_count_; ++i) {
      authors_[i] = new char [strlen(rhl.authors_[i])+1];
      strcpy(authors_[i], rhl.authors_[i]);
    }
  }
  if (rhl.title_) {
    title_ = new char [strlen(rhl.title_)+1];
    strcpy(title_, rhl.title_);
  }
  year_ = rhl.year_;
  return *this;
}
 
void Book::addAuthor(const char* name) {
  if (name==NULL) return;
  
  char ** new_authors = new char* [authors_count_+1];
  if (authors_count_>0)
    memcpy(new_authors, authors_, authors_count_ * sizeof(char*));
  new_authors[authors_count_] =  new char [strlen(name)+1];
  strcpy(new_authors[authors_count_], name);
  char ** tmp = authors_;
  authors_ = new_authors;
 
  if (tmp) {
    delete [] tmp;
  }
  ++authors_count_;
}
 
void Book::setTitle(const char* title) {
  if (title_) {
    delete [] title_;
  }
 
  if (title) {
    title_ = new char [strlen(title)+1];
    strcpy(title_, title);
  } else {
    title_ = NULL;
  }  
}
 
void Book::setYear(const int year) {
  year_ = year;
}
 
char* Book::toString() const {
  size_t result_size = 1;
  for(size_t i=0; i<authors_count_; ++i) {
    result_size += strlen(authors_[i])+2;
  }
  if (title_) {
    result_size += strlen(title_)+2;
  } else {
    result_size += 2;
  }
  result_size += 22; // for year
  
  char * result = new char [result_size];
  size_t l=0;
  size_t n=0;
  for(size_t i=0; i<authors_count_; ++i) {
    l = sprintf(&result[n], "%s, ", authors_[i]);
    n += l;
  }
  if (title_) {
    l = sprintf(&result[n], "%s, ", title_);
    n += l;
  } else {
    l = sprintf(&result[n], ", ");
    n += l;
  }
  sprintf(&result[n], "%d", year_);
  return result;
}
 
int main() {
  Book * book1 = new Book();
  book1->addAuthor("Petrov P.P.");
  book1->addAuthor("Sidorov S.S.");
  book1->setTitle("Hello world");
  book1->setYear(1945);
  char *str = book1->toString();
  std::cout << str << std::endl;
  delete [] str;
 
  Book book2 = Book();
  book2.addAuthor("Ivanov I.I.");
  book2.setTitle("World of C++");
  book2.setYear(2015);
  str = book2.toString();
  std::cout << str << std::endl;
  delete [] str;
  
  Book book3 = *book1;
  str = book3.toString();
  std::cout << str << std::endl;
  delete [] str;
  
  book3 = book2;
  str = book3.toString();
  std::cout << str << std::endl;
  delete [] str;
  
  book3 = book3;
  str = book3.toString();
  std::cout << str << std::endl;
  delete [] str;
 
  delete book1;
  return 0;
}

#include <iostream>
#include <string>
#include <sstream>
#include <vector>
#include <memory>
 
class Book {
  std::vector<std::string> authors_;
  std::string title_;
  int year_;
public:
  Book();
  void addAuthor(const std::string &name);
  void setTitle(const std::string &title);
  void setYear(const int year);
  std::string toString() const;
};
 
Book::Book()
  : authors_(std::vector<std::string>()),
    title_(""),
    year_(0)
{}
 
void Book::addAuthor(const std::string &name) {
  authors_.push_back(name);
}
 
void Book::setTitle(const std::string &title) {
  title_ = title;
}
 
void Book::setYear(const int year) {
  year_ = year;
}
 
std::string Book::toString() const {
  std::stringstream ss;  
  for(auto &author: authors_) {
    ss << author << ", ";
  }
  ss << title_ << ", " << year_;
  return ss.str();
}
 
int main() {
  std::shared_ptr<Book> book1 = std::make_shared<Book>();
  book1->addAuthor("Petrov P.P.");
  book1->addAuthor("Sidorov S.S.");
  book1->setTitle("Hello world");
  book1->setYear(1945);
  std::string str = book1->toString();
  std::cout << str << std::endl;
  
  
  Book book2 = Book();
  book2.addAuthor("Ivanov I.I.");
  book2.setTitle("World of C++");
  book2.setYear(2015);
  str = book2.toString();
  std::cout << str << std::endl;
  
  Book book3 = *book1;
  str = book3.toString();
  std::cout << str << std::endl;
  
  book3 = book2;
  str = book3.toString();
  std::cout << str << std::endl;
  
  book3 = book3;
  str = book3.toString();
  std::cout << str << std::endl;
  
  return 0;
}