Sort list using STL sort function

Question

I'm trying to sort a list (part of a class) in descending order containing 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?

Answer 1

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.

Answer 2

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.

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