Question

I’m trying to write a functor that I can pass to std::sort that will sort a vector representing the indicies for another vector of objects based on the comparison of the objects in the vector. Rather than assume that the objects in the vector have a member operator<, I want to pass in a predicate such as I would pass into std::sort if I was sorting the vector of objects directly.

template <class T, class _Pr>
class SortIndexVectorObjectsHelper : public std::binary_function<ULONG, ULONG, bool>
{
    ULONG m_ulSize;
    std::vector<T> & m_aItems;
    BOOL m_bSortAscending;
    _Pr m_Comp;

public:

    SortIndexVectorObjectsHelper(std::vector<T> & aItems, _Pr Comp) : m_aItems(aItems), m_Comp(Comp), m_ulSize(0)
    {
        m_ulSize = m_aItems.size();
    }

    bool operator()( ULONG & rLeft, ULONG & rRight) 
    {
        if (rLeft < m_ulSize && rRight < m_ulSize)
        {
            T & pLeft = m_aItems[rLeft];
            T & pRight = m_aItems[rRight];

            if (pLeft && pRight)
            {
                return m_Comp(pLeft, pRight);
            }
        }
        return false;
    }


};

struct SortFooByX: public std::binary_function<CFoo,CFoo, bool>
{
    BOOL m_bSortAscending;

    SortFooByX(BOOL bSortAscending)
    {
        m_bSortAscending = bSortAscending;
    }

    bool operator()( CFoo & _Left,  CFoo & _Right) 
    {

         if (m_bSortAscending)
         {
        if (_Left.X() < _Right.X())
           return true;
         }
         else
         {
        if (_Left.X() > _Right.X()) 
                   return true;

          }

          return false;
    }


};

std::sort(aFooIndicies.begin(), aFooIndicies.end(), SortIndexVectorObjectsHelper<CFoo, std::binary_function<CFoo, CFoo, bool> >(aFoo, SortFooByX(FALSE)));

Compiling this gives me the error that none of the 2 overloads could convert all of the argument types.

Answer 1

std::sort(aFooIndicies.begin(), aFooIndicies.end(), SortIndexVectorObjectsHelper<CFoo, std::binary_function<CFoo, CFoo, bool> >(aFoo, SortFooByX(FALSE)));

The second type argument in SortIndexVectorObjectsHelper is wrong. The type should be SortFooByX, as shown below:

SortIndexVectorObjectsHelper<CFoo, SortFooByX> comparer(aFoo, SortFooByX(FALSE));
std::sort(aFooIndicies.begin(), aFooIndicies.end(), comparer);

Try this. It should work now.