Start with a Person class, and create a multiset to hold pointers to person obje

Question

Start with a Person class, and create a multiset to hold pointers to person objects. Define the multiset with the comparePersons function object, so it will be sorted automatically by names of person. Define a half-dozen persons, put them in the multiset, and display its contents. Several of the persons should have the same name, to verify that multiset stores multiple object with the same key.

PROBLEM: I am unable to sort the multiset and I am not sure why.

Code:
// sorts person objects stored by pointer

#include <iostream>
#include <algorithm>
#include <string>
#include <set> //addded---------------------------------------------
using namespace std;

class person
{
private:
   string lastName;
   string firstName;
   long phoneNumber;
public:
   // default constructor
   person() :lastName("blank"), firstName("blank"), phoneNumber(0L)
   { }
   // 3-arg constructor
   person(string lana, string fina, long pho) :
       lastName(lana), firstName(fina), phoneNumber(pho)
   { }
   friend bool operator<(const person&, const person&);
   friend bool operator==(const person&, const person&);

   void display() const // display person's data
   {
       cout << endl << lastName << ", " << firstName
           << " Phone: " << phoneNumber;
   }
   long get_phone() const // return phone number
   {
       return phoneNumber;
   }
}; //end class person
//--------------------------------------------------------------
// overloaded < for person class
bool operator<(const person& p1, const person& p2)
{
   if (p1.lastName == p2.lastName)
       return (p1.firstName < p2.firstName) ? true : false;
   return (p1.lastName < p2.lastName) ? true : false;
}
//--------------------------------------------------------------
// overloaded == for person class
bool operator==(const person& p1, const person& p2)
{
   return (p1.lastName == p2.lastName &&
       p1.firstName == p2.firstName) ? true : false;
}
//--------------------------------------------------------------
// function object to compare persons using pointers
class comparePersons
{
public:
   bool operator() (const person* ptrP1, const person* ptrP2) const
   {
       return *ptrP1 < *ptrP2;
   }
};
//--------------------------------------------------------------
//function object to display a person, using a pointer
class displayPerson
{
public:
   void operator() (const person* ptrP) const
   {
       ptrP->display();
   }
};
////////////////////////////////////////////////////////////////
int main()
{   
   //make persons
   person* ptrP1 = new person("KuangThu", "Bruce", 4157300);
   person* ptrP2 = new person("Deauville", "William", 8435150);
   person* ptrP3 = new person("Wellington", "John", 9207404);
   person* ptrP4 = new person("Bartoski", "Peter", 6946473);
   person* ptrP5 = new person("Fredericks", "Roger", 7049982);
   person* ptrP6 = new person("McDonald", "Stacey", 7764987);
   person* ptrP7 = new person("KuangThu", "Bruce", 4157300);
   person* ptrP8 = new person("Deauville", "William", 8435150);

   //Creating multiset
   multiset<person*> multiPtrsPers;
   multiPtrsPers.insert(ptrP1);
   multiPtrsPers.insert(ptrP2);
   multiPtrsPers.insert(ptrP3);
   multiPtrsPers.insert(ptrP4);
   multiPtrsPers.insert(ptrP5);
   multiPtrsPers.insert(ptrP6);
   multiPtrsPers.insert(ptrP7);
   multiPtrsPers.insert(ptrP8);

   for_each(multiPtrsPers.begin(),multiPtrsPers.end(), displayPerson());//display person
   cout << endl;

   sort(multiPtrsPers.begin(), multiPtrsPers.end(), comparePersons());//compare persons

   for_each(multiPtrsPers.begin(), multiPtrsPers.end(), displayPerson());//display persons
   cout << endl;

   system("pause");
   return 0;
} // end main()

Explanation / Answer

That is some real good work you did there. But you missed one point, multiset sorts automatically i.e. the class comparePersons you used to sort multiset, it is used in construction of multiset, and multiset uses this class to keep its elements in sorted fashion in some container. Enough said, following is that code:

#include <iostream>
#include <algorithm>
#include <string>
#include <set> //addded---------------------------------------------
using namespace std;

class person
{
private:
    string lastName;
    string firstName;
    long phoneNumber;
public:
    // default constructor
    person() :lastName("blank"), firstName("blank"), phoneNumber(0L)
    { }
    // 3-arg constructor
    person(string lana, string fina, long pho) :
        lastName(lana), firstName(fina), phoneNumber(pho)
    { }
    friend bool operator<(const person&, const person&);
    friend bool operator==(const person&, const person&);

    void display() const   // display person's data
    {
        cout << endl << lastName << ", " << firstName
            << " Phone: " << phoneNumber;
    }
    long get_phone() const // return phone number
    {
        return phoneNumber;
    }
}; //end class person
   //--------------------------------------------------------------
   // overloaded < for person class
bool operator<(const person& p1, const person& p2)
{
    if (p1.lastName == p2.lastName)
        return (p1.firstName < p2.firstName) ? true : false;
    return (p1.lastName < p2.lastName) ? true : false;
}
//--------------------------------------------------------------
// overloaded == for person class
bool operator==(const person& p1, const person& p2)
{
    return (p1.lastName == p2.lastName &&
        p1.firstName == p2.firstName) ? true : false;
}
//--------------------------------------------------------------
// function object to compare persons using pointers
class comparePersons
{
public:
    bool operator() (const person* ptrP1, const person* ptrP2) const
    {
        return *ptrP1 < *ptrP2;
    }
};
//--------------------------------------------------------------
//function object to display a person, using a pointer
class displayPerson
{
public:
    void operator() (const person* ptrP) const
    {
        ptrP->display();
    }
};

////////////////////////////////////////////////////////////////
int main()
{                       
    //make persons
    person* ptrP1 = new person("KuangThu", "Bruce", 4157300);
    person* ptrP2 = new person("Deauville", "William", 8435150);
    person* ptrP3 = new person("Wellington", "John", 9207404);
    person* ptrP4 = new person("Bartoski", "Peter", 6946473);
    person* ptrP5 = new person("Fredericks", "Roger", 7049982);
    person* ptrP6 = new person("McDonald", "Stacey", 7764987);
    person* ptrP7 = new person("KuangThu", "Bruce", 4157300);
    person* ptrP8 = new person("Deauville", "William", 8435150);

    //Creating multiset
    multiset<person*, comparePersons> multiPtrsPers;
    multiPtrsPers.insert(ptrP1);
    multiPtrsPers.insert(ptrP2);
    multiPtrsPers.insert(ptrP3);
    multiPtrsPers.insert(ptrP4);
    multiPtrsPers.insert(ptrP5);
    multiPtrsPers.insert(ptrP6);
    multiPtrsPers.insert(ptrP7);
    multiPtrsPers.insert(ptrP8);

    for_each(multiPtrsPers.begin(),multiPtrsPers.end(), displayPerson());//display person
    cout << endl;

/*    sort(multiPtrsPers.begin(), multiPtrsPers.end(), comparePersons());//compare persons

    for_each(multiPtrsPers.begin(), multiPtrsPers.end(), displayPerson());//display persons
    cout << endl;
*/
    system("pause");
    return 0;
} // end main()

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