Why Can't I Sort a List in Descending Order Using `std::sort` and a Custom Comparator?

Sorting Lists Using STL Sort Function

Implementing list sorting in descending order can be challenging when the list contains objects of a custom struct. However, the issue you encountered, "no match for 'operator-' in '__last - __first'", reveals a deeper problem with the chosen sorting approach.

The provided SortDescending struct correctly defines an operator to compare two terms, but it does so based on their power, t2.pow < t1.pow. This comparison, however, is incompatible with the standard sort function.

The problem stems from the fact that std::sort requires random access iterators, which list iterators do not provide. Bidirectional iterators, like those in std::list, allow you to move through the list, but they lack the necessary capabilities to perform direct access or pointer arithmetic.

To sort a list using the STL sort function, random access iterators must be available. An alternative approach is to use the member function std::list::sort. This function accepts a comparator function as an argument, enabling you to define a customized sorting criteria.

In your case, the following code will sort the list in descending order based on the power of its terms:

Result.poly.sort([](const term& t1, const term& t2) { return t1.pow > t2.pow; });

By using the correct iterators and appropriate sorting function, you can effectively sort your list in the desired order.
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