C++ Sort class array using qsort - c++

Question

C++:
I'm trying to sort some students that are stored in a class by average media.
Only qsort, don't advice me of std::sort, thank you!
Qsort compare function:
int cmp(Student *a, Student *b) {
return (int)(((Student *)b)->get_media() - ((Student *)a)->get_media());
}
qsort call:
qsort(&tab, (size_t)n, sizeof(tab), (int(*)(const void*, const void*))cmp);
There's no compiler error, but it won't sort.
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
using namespace std;
class Student {
private:
char name[20];
char surname[20];
int *marks;
int group;
float avg_mark;
public:
Student()
{
char na[20], sur[20];
int group;
cout << "\nEnter name: ";
cin >> na;
cout << "\nEnter surname: ";
cin >> sur;
cout << "\nEnter group: ";
cin >> group;
init(na, sur, group);
}
~Student()
{
cout << "\ndestructor";
delete []marks;
}
void init(char *n, char *p, int gr)
{
strcpy(name, n);
strcpy(surname, p);
group = gr;
marks = new int[6];
for (int i = 0; i < 6; i++)
{
cout << "\nEnter mark " << i + 1 << ": ";
cin >> *(marks + i);
}
avg_mark = media();
}
float media()
{
int s = 0;
for (int i = 0; i < 6; i++)
s += marks[i];
return ((float)s / 6);
}
void set_name(char *n)
{
strcpy(name, n);
}
char* get_name()
{
return name;
}
void set_surname(char *p)
{
strcpy(name, p);
}
char* get_surname()
{
return surname;
}
int get_group()
{
return group;
}
float get_media()
{
return avg_mark;
}
};
int cmp(Student *a, Student *b);
int comparator(void *a, void *b) {
return (int)(((Student *)b)->get_media() - ((Student *)a)->get_media());
}
void main(void)
{
int n;
cout << "\nEnter n: ";
cin >> n;
Student *tab = new Student[n];
for (int i = 0; i < n; i++)
cout << i + 1 << ". " << tab[i].get_name() << " " << tab[i].get_surname() << " Group:" << tab[i].get_group() << " Average mark: " << tab[i].get_media() << endl;
//qsort(&tab[0], (size_t)n, sizeof(tab), (int*)cmp);
cout << endl;
qsort(&tab, (size_t)n, sizeof(tab), (int(*)(const void*, const void*))cmp);
for (int i = 0; i < n; i++)
cout << i + 1 << ". " << tab[i].get_name() << " " << tab[i].get_surname() << " Group:" << tab[i].get_group() << " Average mark: " << tab[i].get_media() << endl;
cin.ignore();
cin.get();
}
int cmp(Student *a, Student *b) {
return (int)(((Student *)b)->get_media() - ((Student *)a)->get_media());
}

Answer 1

qsort(&tab, (size_t)n, sizeof(tab), (int(*)(const void*, const void*))cmp);
&tab is the address of the pointer tab. You want to pass the address of the first element of your array. That's &tab[0] or simply tab.
Also, you need to pass the size of a Student object, not the size of a pointer. So change sizeof(tab) to sizeof(Student) or sizeof(*tab). So the call should look like this:
qsort(tab, (size_t)n, sizeof(*tab), (int(*)(const void*, const void*))cmp);