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?

Thanks!