Question

I’m finding that std::sort is very slow with sorting only 1000 items.

In class template template <typename T> class TableModel : public QAbstractTableModel I have the following function to sort a table.

template<typename T>
void TableModel<T>::sort(int column, Qt::SortOrder order = Qt::AscendingOrder) {
    if(order == Qt::AscendingOrder) {
        qSort(m_list.begin(), m_list.end(), less<T>(column));
    } else {
        qSort(m_list.begin(), m_list.end(), greater<T>(column));
    }
    reset();
 }

I notice if I only have the randomly shuffle my table is shuffles then displays instantly. So this leads me to think that its sort that is slow. Can anyone help me speed up the sorting of a QTable?

Here is the less struct.

template<typename T>
struct less {
    int index;
    less(int index) : index(index) {}
    bool operator()(const T& first, const T& second) {
        return T::less(first, second, index);
    }
};

T::less is a function and all it does it the less than comparison based on the index given.

Slow is defined as a 5 seconds for only 1000 items when I need to handle about 100,000 items later on.

Answer 1

I suspect that m_list is storing the items by value and that swapping them is expensive. You could try to either implement a faster swap or store them in the container by smart pointer.

Of course a profiler could help you pinpoint the problem much more precisely.