C++ Lottery Machine implementation

lotto.c++

#include <cstddef>

#include <numeric>
#include <random>
#include <stdexcept>
#include <vector>

class LotteryMachine {
public:
    LotteryMachine(std::size_t number_of_balls)
      : _prng(std::random_device()())
      , _balls(number_of_balls)
      {
        std::iota(_balls.begin(), _balls.end(), 1);
    }
    std::size_t pick_ball() {
        if (_balls.empty()) throw std::runtime_error("No balls left");
        std::uniform_int_distribution<std::size_t> gen(0, _balls.size() - 1);
        auto pick = _balls.begin() + gen(_prng);
        std::size_t ball = *pick;
        _balls.erase(pick);
        return ball;
    }
private:
    std::default_random_engine _prng;
    std::vector<std::size_t> _balls;
};

#include <iomanip>
#include <iostream>

int main() {
    LotteryMachine lotto(59);
    for (int j = 0; j < 10; ++j) {
        for (int i = 0; i < 6; ++i) {
            std::cout << std::setfill('0') << std::setw(2) << lotto.pick_ball() << " ";
            std::cout.flush();
        }
        std::cout << std::endl;
    }
}