Sort list using stl sort function

Question

I'm trying to sort a list (part of a class) in descending containg items of a struct but it doesn't compile(error: no match for 'operator-' in '__last - __first'):

sort(Result.poly.begin(), Result.poly.end(), SortDescending());

And here's SortDescending:

 struct SortDescending
{
    bool operator()(const term& t1, const term& t2)
    { 
        return t2.pow < t1.pow; 
    }
};

Can anyone tell me what's wrong?

EDIT It works now thanks to Glen and everyone else!

Thanks!

Answer 1

std::list has a built-in sort method that you need to use since std::sort only works with random access iterators, whereas std::list::iterator merely belongs to the bidirectional iterator class of iterators.

But you don’t need to write your own comparer for that, simply use std::greater<T>:

Result.poly.sort(comparer, SortDescending());

Also, your operator () should be marked const.

struct SortDescending
{
    bool operator()(const term& t1, const term& t2) const
    { 
        return t2.pow < t1.pow; 
    }
};

Finally, you don’t need to write your own comparer for that, simply use std::greater<T> (located in the standard header <functional>):

Result.poly.sort(greater<term>());

Answer 2

It seems like the iterator types for Result.poly is missing operator -. std::sort doesn't work with std::list change to Result.poly.sort

Answer 3

The standard algorithm std::sort requires random access iterators, which std::list<>::iterators are not (list iterators are bidirectional iterators).

You should use the std::list<>::sort member function.