Using CListCtrl::SortItems

In my previous article, I explained how to use LPSTR_TEXTCALLBACK and LVN_GETDISPINFO.

This article will show how to use SortItems to sort the items in the list.

Why would we want to do this? If we’d just stored the text in the list control, rather than use LPSTR_TEXTCALLBACK, it would have sorted for us.

Well, yes it would, but it would have been wrong. In the last article, we used a variety of functions (e.g. StrFormatKBSize) to produce the strings that went into the list control. When sorting, we’d prefer to order the underlying values, not the formatted text.

This is where SortItems comes in.

Calling SortItems

When you call SortItems, you need to pass a comparison function, like this:

int CALLBACK CompareFunction(LPARAM lParam1, LPARAM lParam2, LPARAM lParamSort);

lParam1 and lParam2 are the lParam values from the two items to be compared. lParamSort is user-defined data passed to SortItems. The return value of your comparison function should be like that of strcmp. That is: it should return a negative value if lhs < rhs, a positive value if lhs > rhs, or zero if lhs is equal to rhs. Our comparison function looks like this:

int CALLBACK CompareFunction(LPARAM lParam1, LPARAM lParam2, LPARAM lParamSort)
{
    int iSubItem = (int)lParamSort;

    const FileItem *item1 = reinterpret_cast<const FileItem *>(lParam1);
    const FileItem *item2 = reinterpret_cast<const FileItem *>(lParam2);

    switch (iSubItem)
    {
    case SUBITEM_NAME:
        {
            CString name1 = item1->GetName();
            CString name2 = item2->GetName();

            return name1.CompareNoCase(name2);
        }

    case SUBITEM_SIZE:
        {
            LONGLONG size1 = item1->GetSize();
            LONGLONG size2 = item2->GetSize();

            return (size1 == size2) ? 0 : (size1 < size2) ? -1 : 1;
        }

    case SUBITEM_MODIFIED:
        {
            FILETIME mod1 = item1->GetModified();
            FILETIME mod2 = item2->GetModified();

            return CompareFileTime(&mod1, &mod2);
        }
    }

    return 0;
}

As you can see, we can use the same function for sorting on each of the different columns if we pass the sub-item ID in lParamSort. This function also demonstrates how to compare different types of data. Note that the string comparison is not technically correct: it doesn’t take into account locale settings.

We call it from our HDN_ITEMCLICK handler:

void CListSortDlg::OnHdnItemclickListctrl(NMHDR *pNMHDR, LRESULT *pResult)
{
    LPNMHEADER phdr = reinterpret_cast<LPNMHEADER>(pNMHDR);

    int nCol = phdr->iItem;

    LVCOLUMN column;
    memset(&column, 0, sizeof(LVCOLUMN));
    column.mask = LVCF_SUBITEM;
    VERIFY(m_listCtrl.GetColumn(nCol, &column));
    int iSubItem = column.iSubItem;

    // We need to sort by iSubItem.
    VERIFY(m_listCtrl.SortItems(CompareFunction, iSubItem));

    *pResult = 0;
}