sorting vector object of Student based on last first id

Question

this is the string to compare, lastname+firstName and id comp is public, my sort method does not work, I am not able to find where the code fails, I am supposed to have the students listed in the order

comp=new char[strlen(lName)+strlen(fName)+strlen(id)+1];

this is my student constructor:

Student::Student(char * first, char * last, char *i,char * stand, int credit, double g, Date * d, Date * matricDate){
        lName=last;
        fName=first;
        id=i;
        standing=stand;
        credits=credit;
        gpa=g;
        dob=d;
        matDate=matricDate;
        comp=new char[strlen(lName)+strlen(fName)+strlen(id)+1];
        sprintf(comp,"%s%s%s",lName,fName,id);
};

this is the operator I overload

 bool Student::operator<(const Student &second){
                if(comp<second.comp){
                        return true;
                }else{
                        return false;
                }
        };

I have the student list as:

 vector<Student *> roster_list;

I add student like this:

void Roster::addStudent(Student *student){

        roster_list.push_back(student); 

        };

here is my sort:

void Roster::mySort(){
        sort(roster_list.begin(),roster_list.end());
};

my main:

#include<iostream>
#include"Roster.cpp"
#include "Date.cpp"
#include "Student.cpp"

using namespace std;


int main(){

        Roster *r=new Roster();
        Date *d=new Date(12,05,1987);
        Student *st=new Student("Jason", "Sam", "124542", "sophomore", 34, 4.0, d,d);
        Student *st2=new Student("Ahmad", "Khan", "23452", "freshman", 34, 4.0, d,d);

        Student *st3=new Student("Selam", "Can", "23431", "freshman", 34, 3.0, d,d);
        Student *st4=new Student("Andrew", "Rosenberg", "34523", "Senior", 34, 4.0, d,d);
        Student *st5=new Student("Selam", "Can", "23431", "freshman", 34, 3.0, d,d);


        r->addStudent(st);
        r->addStudent(st2);
        r->addStudent(st3);
        r->addStudent(st4);

        // Student 3 and 5 are same 
        // == operator check
        if(*st3==*st5){
                cout<<"Students are same \n";
        }

        // != overloading operator check
        if(*st3!=*st4){
                cout<<"Students are different\n";
        }

        cout<<"################Before Sorting###############\n";
        r->toString();
        cout<<"################After Sorting###############\n";
        r->mySort();
        r->toString();



        cout<<"\n"<<st->getComp();
        cout<<"\n"<<st3->getComp();

        return 0;
}

OUTPUT:

Students are same 
Students are different
################Before Sorting###############
Student Info:Jason      Sam     124542  sophomore       4
Birth Date: December 5, 1987
Matric Date: December 5, 1987
Student Info:Ahmad      Khan    23452   freshman        4
Birth Date: December 5, 1987
Matric Date: December 5, 1987
Student Info:Selam      Can     23431   freshman        3
Birth Date: December 5, 1987
Matric Date: December 5, 1987
Student Info:Andrew     Rosenberg       34523   Senior  4
Birth Date: December 5, 1987
Matric Date: December 5, 1987
################After Sorting###############
Student Info:Jason      Sam     124542  sophomore       4
Birth Date: December 5, 1987
Matric Date: December 5, 1987
Student Info:Ahmad      Khan    23452   freshman        4
Birth Date: December 5, 1987
Matric Date: December 5, 1987
Student Info:Selam      Can     23431   freshman        3
Birth Date: December 5, 1987
Matric Date: December 5, 1987
Student Info:Andrew     Rosenberg       34523   Senior  4
Birth Date: December 5, 1987
Matric Date: December 5, 1987

SamJason124542
CanSelam23431

I tried this

bool StudentSort(Student* lhs, Student* rhs) {
    return (*lhs)<(*rhs);
}



void Roster::mySort(){
        sort(roster_list.begin(),roster_list.end(),StudentSort);
};

still does not work, I get compilation error I have gcc version 4.4.7 20120313 (Red Hat 4.4.7-3) (GCC)

this is the error I get after this try:

Roster.cpp: In member function ‘void Roster::mySort()’:
Roster.cpp:69: error: argument of type ‘bool (Roster::)(Student*, Student*)’ does not match ‘bool (Roster::*)(Student*, Student*)’

Answer 1

Your vector/list contains only Student* , not Student's. So, you would need to provide Std::sort something that compares Student* correctly. As it stands, default sort will probably being sorting by memory location, not the semantic value of the pointed to object.

// assumes operator< above,which it doesnt.
bool StudentSortFn(Student* lhs, Student* rhs) {
    return (*lhs)<(*rhs);
}

Of course, it turns out the original operator< has much the same issue embedded in it. Its comparing char* fields based on pointer values (memory addresses), not on semantic contents. It need to be rewritten to use strcmp or something similar like:

 bool Student::operator<(const Student &second){
        return strcmp(comp,second.comp)<0;
    };

Answer 2

It's hard to sort Students when your vector is a vector of pointers. Simple way is to rewrite without pointers

vector<Student> roster_list;

void Roster::addStudent(Student student){

    roster_list.push_back(student); 

    };

int main(){

    Roster r;
    Date d(12,05,1987);
    Student st("Jason", "Sam", "124542", "sophomore", 34, 4.0, d,d);
    Student st2("Ahmad", "Khan", "23452", "freshman", 34, 4.0, d,d);

    Student st3("Selam", "Can", "23431", "freshman", 34, 3.0, d,d);
    Student st4("Andrew", "Rosenberg", "34523", "Senior", 34, 4.0, d,d);
    Student st5("Selam", "Can", "23431", "freshman", 34, 3.0, d,d);

    r.addStudent(st);
    r.addStudent(st2);
    r.addStudent(st3);
    r.addStudent(st4);

etc. etc.