Sorting an std::list that contains structs

Question

So I'm having some trouble figuring out the proper use of list::sort() in regards to a list of structs. Heres the relevant code:

struct student
{
    char firstnm[20],   
        lastnm[20];     
    int id,             
        grade;
};
list<student> sList;

//Irrelevant code...

cout << "Please enter your own name, id, and grade. (Ex: myfirst mylast 0 12)\n";
cin >> data.firstnm >> data.lastnm >> data.id >> data.grade;
sList.push_back(data);
sList.sort();

The problem I'm trying to solve is using sList.sort() to sort by id . However, I have no idea how to properly pass it into list::sort() . Thanks in advance for any help/time!

EDIT: The solution was simply adding this to my struct

bool operator < (const student& cmp) const {
   return id < cmp.id;
}

Answer 1

The member function std::list::sort() is overloaded. std::list has an ordinary member function that takes no parameters and uses operator< for sorting the elements of the list:

void sort();

It also has a member function template taking one parameter comp that is used as the predicate for sorting the elements:

template<class Compare> 
void sort(Compare comp);

---

Assuming you want to sort the student objects in your list according to the key id in ascending order . You can either define operator< for your user-defined student class as:

bool operator<(student const& a, student const& b) {
   return a.id < b.id;
}

and use the overload that takes no parameters:

sList.sort(); // 1st overload

or simply plass a lambda expression to the member function template taking one parameter:

auto cmp = [](auto const& a, auto const& b) {
   return a.id < b.id;
}; 

sList.sort(cmp); // 2nd overload

with this approach, you don't need to define operator< for your student class.

---

Note that you can't use the std::sort() algorithm on std::list 's iterators, because this algorithm requires random-access iterators, but an std::list only provides bidirectional iterators. Therefore, the following code won't compile:

std::sort(sList.begin(), sList.end());

Instead, you have to use the member functions provided by std::list in order to sort the elements of the container as explained above.

Answer 2

You should give a look to std::sort. ( https://en.cppreference.com/w/cpp/algorithm/sort ) There is multiple definitions of that function, and one where you can specify on what you want to sort.

Also, give a look to that post, I think it's what you need : https://stackoverflow.com/a/21234017/6663947

Edit :

thats an exemple of comparator :

sList.sort([](const student & a, const student & b) { return a.id < b.id; });

I did not tried it, but it should look like it. Also, this is for c++11

Hope it helps!