Segmentaion fault (core dumped) error using qsort

[tanveer00]

I am getting a segmentation fault (core dumped) error using qsort.
Here are the structures I am using:

struct team_info
{
    string name;
    int played, won, drawn, lost, points, goals_for, goals_against, diff;
};
struct tourney
{
    string tourney_name;
    int n_teams;
    int n_games;
    struct game
    {
        string home;
        int h_score;
        string away;
        int a_score;
    }g[1000];
    struct team_info team[30];
}t;

Here is the compare function I am using to compare the "points" field in struct team_info:

int compare_function_points(const void *a, const void *b)
{
    const struct team_info *sa = (struct team_info *)a;
    const struct team_info *sb = (struct team_info *)b;

    return sa->points - sb->points;
}

Here, is the qsort() call in my main() function:

qsort(t.team, t.n_teams, sizeof(t.team), compare_function_points);

[MiiNiPaa]

1) You shall not use qsort for non-POD types. It wreaks their integrity. Also there is a great chance that you forgot to initialize n_teams. Is there any reason to use C function qsort instead of C++ std::sort?

2) Any reason to explicitely place struct before typename in C++?

You are trying to write C in C++. Do not do that.

[tanveer00]

Thanks for the reply.
What do you mean by non-POD types (excuse me, I am a beginner)?
I did initialize n_teams in the main() function.
Our teacher told us we could use qsort to sort an array of structs. I still did not get the reason not to use qsort in this case.
How could I use C++ std::sort in this case, that is, if I had to sort struct team_info teams[] by their "points"?
Is there any problem if I explicitly place struct before typename in C++?
Thanks in advance. :)

[MiiNiPaa]

Is there any problem if I explicitly place struct before typename in C++?

No, but there is no need to do so in C++. Usually this is the first sign that person does not actually learning C++, but C with classes instead.

How could I use C++ std::sort in this case, that is, if I had to sort struct team_info teams[] by their "points"?

Same way like you would use qsort: create comparison function and pass it to std::sort. But without type erasure, unsafe cast and with compiler error checking:

#include <algorithm>
#include <iostream>
#include <string>

struct team_info
{
    std::string name;
    int played, won, drawn, lost, points, goals_for, goals_against, diff;
};

struct tourney
{
    std::string tourney_name;
    int n_teams;
    int n_games;
    struct game
    {
        std::string home;
        int h_score;
        std::string away;
        int a_score;
    }g[1000];
    team_info team[30];
} t;

bool compare_points_less(const team_info& lhs, const team_info& rhs)
{
    return lhs.points < rhs.points;
}

int main()
{
    const int teams = 5;
    t.n_teams = teams;
    //initialize teams
    int scores[teams] = { 1, 8, 3, 5, 0};
    for(int i = 0; i < teams; ++i) {
        t.team[i].points = scores[i];
        std::cout << t.team[i].points << ' ';
    }
    std::cout << '\n';

    //sort
    std::sort(t.team, t.team + t.n_teams, compare_points_less);
    
    //display sorted
    for(int i = 0; i < t.n_teams; ++i) {
        std::cout << t.team[i].points << ' ';
    }
}

Our teacher told us we could use qsort to sort an array of structs.

Only trivial types like int. double, etc. string is not a trivial type and should not be sorted that way. Often strings contain a so-called small string optimisation which required them to contain pointer to itself. If you do a bitwise swap, those pointers start pointing to other object, completely breaking class and making it unusable and dangerous.

What do you mean by non-POD types

http://stackoverflow.com/questions/4178175/what-are-aggregates-and-pods-and-how-why-are-they-special/7189821#7189821
std::string is not trivial, so whole structure is not trivial either.

Edit: also standard explicitely says that qsort works only on trivial types:

The type of the elements of the array must be a trivial type, otherwise the behavior is undefined.

[tanveer00]

MiNiPaa, I can NOT thank you enough! I've been stuck with this all night.
There is one little question though.

bool compare_points_less(const team_info& lhs, const team_info& rhs)

I can't get why you used the ampersand (&) operator before lhs, and rhs?

[MiiNiPaa]

This is passing by reference. Instead of copy of object we pass an object itself. Think about it as of safer pointer which does not need dereference
http://www.cplusplus.com/doc/tutorial/functions/#reference
http://www.learncpp.com/cpp-tutorial/73-passing-arguments-by-reference/

[saadusmani]

These ampersand(&) are used when we have to pass arguments by reference.