He's using integers as sets of (small) integers,

    int Possibles = Block[BlockIndex] & Row[RowIndex] & Col[ColIndex];
    while (Possibles) {
        int valbit = Possibles & (-Possibles); // Lowest 1 bit in Possibles
        Possibles &= ~valbit;
        Entry[Square] = valbit;
        Block[BlockIndex] &= ~valbit;
        Row[RowIndex] &= ~valbit;
        Col[ColIndex] &= ~valbit;

is more generally:

    set<int> possibles = block[block_index].intersect(row[row_index]).intersect(col[col_index]);
    while (!possibles.is_empty())
    {
        int value = possibles.pop();
        entry[square] = value;
        block[block_index].remove(value);
        row[row_index].remove(value);
        col[col_index].remove(value);