using iterator in ostream fails - c++

Question

I am attempting to implement a std::list to replace a linked list in this assignment. I am not allowed to change the declarations and can only change code in the .cpp file. For the most part I am making progress but I am having trouble implementing this
std::ostream& operator<< (std::ostream& out, const Section& section);
namely when I try to create an iterator it fails. I've used the iterator elsewhere in the code so I don't understand why it's failing here, I believe it's because it's private but I'm not sure how to resolve the issue without changing the .h file which was explicitly prohibited:
std::ostream& operator<< (std::ostream& out, const Section& section)
{
// 1. print the section header
out << setw(8) << left << section.getCourse()
<< setw(6) << left << section.getCallNumber();
out << ": " << section.getNumberOfStudents() << " students\n";
// 2. collect the students, sort, and print
Student* students = new Student[section.getNumberOfStudents()];
{
int i = 0;
for ( auto pos = section.students.begin();
pos != section.students.end(); pos++)
{
students[i] = pos;
++i;
}
}
sort (students, students+section.getNumberOfStudents());
for (int i = 0; i < section.getNumberOfStudents(); ++i)
out << " " << students[i] << "\n";
out << flush;
return out;
}

Answer 1

students[i] = pos;
should be changed to
students[i] = *pos;
because you want to copy the Student the iterator references, not the iterator itself.
But why a dynamic array of Student rather than a std::vector<Student>? Currently you have a memory leak because you don't delete[] students;
Edit 1
Removed.
Edit 2
Other than that, all I can see that it wrong is a missing std:: in front of
sort (students, students+section.getNumberOfStudents());
this is assuming there is no custom sort method being used.
Edit 3
Going off the rails here:
students[i] = *pos;
copies a Student from the list into the dynamic array students. This could be expensive, so here is an alternative:
First the bits and pieces needed to prove this out: Required includes
#include <iostream>
#include <list>
#include <vector>
#include <algorithm>
#include <functional>
a minimal Student class
class Student
{
std::string name;
public:
Student(std::string inname):name(inname)
{
}
const std::string & getname() const
{
return name;
}
friend bool operator<(const Student & a, const Student &b)
{
return a.name < b.name;
}
};
a minimal Section class
class Section
{
public:
std::list<Student> students;
};
a minimal outstream operator
std::ostream& operator<<(std::ostream& out, const Section& section)
{
A std::vector instead of an array, and a vector of constant references so we don't have to copy the students.
std::vector<std::reference_wrapper<const Student>> students;
Store references in the vector. Probably could do a one liner with std::copy and std::back_inserter, but this is getting a bit too much to absorb for one example.
for (const auto & student: section.students)
{
students.push_back(std::ref(student));
}
Sort the vector
std::sort(students.begin(), students.end());
print the vector
for (const auto & student: students)
{
out << student.get().getname() << " ";
}
return out;
}
and one main to rule them all and in the darkness bind them
int main()
{
Section s;
s.students.emplace_front("Tom");
s.students.emplace_front("Dick");
s.students.emplace_front("Harry");
std::cout << s;
}
And all in one easy to cut-n-paste block:
#include <iostream>
#include <list>
#include <vector>
#include <algorithm>
#include <functional>
class Student
{
public:
std::string name; // this is me being lazy. name should be private
Student(std::string inname):name(inname)
{
}
const std::string & getname() const
{
return name;
}
friend bool operator<(const Student & a, const Student &b)
{
return a.name < b.name;
}
};
class Section
{
public:
std::list<Student> students;
};
std::ostream& operator<<(std::ostream& out, const Section& section)
{
std::vector<std::reference_wrapper<const Student>> students;
// store references in the `vector`.
for (const auto & student: section.students)
{
students.push_back(std::ref(student));
}
// Sort the `vector`
std::sort(students.begin(), students.end());
// print the `vector`
for (const auto & student: students)
{
out << student.get().getname() << " ";
}
return out;
}
int main()
{
Section s;
s.students.emplace_front("Tom");
s.students.emplace_front("Dick");
s.students.emplace_front("Harry");
std::cout << s;
}
Or do what Remy suggested and use a std::vector<Student *> and a custom comparator to dereference the pointers for std::sort.

Answer 2

As others have stated, the error is because you are not dereferencing the iterator when populating your students[] array:
students[i] = pos; // <-- should be *pos instead!
I would suggest an alternative approach that should be faster and more efficient:
std::ostream& operator<< (std::ostream& out, const Section& section)
{
// 1. print the section header
out << setw(8) << left << section.getCourse()
<< setw(6) << left << section.getCallNumber();
out << ": " << section.getNumberOfStudents() << " students\n";
// 2. collect the students, sort, and print
std::vector<const Student*> students;
students.reserve(section.getNumberOfStudents());
for ( auto pos = section.students.cbegin();
pos != section.students.cend(); ++pos)
{
students.push_back(&(*pos));
}
sort (students.begin(), students.end(),
[](const Student *a, const Student *b) { return (*a < *b); }
);
for ( auto pos = students.cbegin();
pos != students.cend(); ++pos)
{
out << " " << *(*pos) << "\n";
}
out << flush;
return out;
}

Answer 3

I appreciate all your answers. Ended up being a much more basic issue. I had to implement the Section iterators to return student iterators.
Section::iterator Section::begin() {
return students.begin();
}
Section::const_iterator Section::begin() const {
return students.begin();
}
Section::iterator Section::end() {
return students.begin();
}
Section::const_iterator Section::end() const {
return students.begin();
}