C++ | 素数ジェネレータの実装例

#include <iostream>
#include <vector>

class PrimeNumberGenerator
{
public:
    PrimeNumberGenerator() : ps_(std::vector<unsigned long>{2}) {}

    unsigned long operator()()
    {
        auto curr = ps_.back();
        auto next = curr + 1;
        while (true)
        {
            if (is_prime(next))
            {
                ps_.push_back(next);
                return curr;
            }
            ++next;
        }
    }

    bool is_prime(unsigned long n) const
    {
        for (auto p : ps_)
        {
            if (p * p > n)
            {
                return true;
            }
            if (n % p == 0)
            {
                return false;
            }
        }
        throw std::runtime_error("unexpected error");
    }

private:
    std::vector<unsigned long> ps_;
};

int main()
{
    PrimeNumberGenerator png;
    for (std::size_t i = 0; i < 100; ++i)
    {
        std::cout << png() << ", ";
    }
    std::cout << std::endl;
    return 0;
}

$ g++ prime.cc && ./a.out
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541,