i != boost::dynamic_bitset<>::npos; 这行代码是什么意思？

void StudyWidgets::GetPrime(int num)
{
    boost::dynamic_bitset<> db(num);//0000000000
    db.set();//1111111111
 
    for (auto i = db.find_next(1); i != boost::dynamic_bitset<>::npos; i = db.find_next(i))
    {
        for (auto j = db.find_next(i); j != boost::dynamic_bitset<>::npos; j = db.find_next(j))
        {
            if (j%i == 0)
            {
                db[j] = 0;
            }
        }
        //索引：  9 8 7 6 5 4 3 2 1 0   
        //i = 2时:1 0 1 0 1 0 1 1 1 1
        //i = 3时:0 0 1 0 1 0 1 1 1 1
        //i = 5时:0 0 1 0 1 0 1 1 1 1
        //i = 7时:0 0 1 0 1 0 1 1 1 1
        std::cout << std::endl << "i=" << i << "时:";
        for (size_t m = db.size() - 1; m >= 0 && m < db.size(); m--)//注意条件，因为当m=0时，m--后 m=18446744073709551615
        {
            std::cout << db[m] << " ";
        }
    }
    std::cout << std::endl;
    //0010101111
    for (auto i = db.find_next(1); i != boost::dynamic_bitset<>::npos; i = db.find_next(i))
    {
        std::cout << i;   //2 3 5 7
    }
}