The point of having a seed_seq is to increase the entropy of the generated sequence. If you have a random_device on your system, initializing with multiple numbers from that random device may arguably do that. On a system that has a pseudo-random number generator I don’t think there is an increase in randomness, i.e. generated sequence entropy.
Building on that your approach:
If your system does provide a random device then you can use it like this:
std::random_device r;
// std::seed_seq ssq{r()};
// and then passing it to the engine does the same
default_random_engine eng{r()};
uniform_real_distribution<double> urd(0, 1);
cout << "Uniform [0, 1): " << urd(eng);
If your system does not have a random device then you can use time(0) as a seed to the random_engine
default_random_engine eng{static_cast<long unsigned int>(time(0))};
uniform_real_distribution<double> urd(0, 1);
cout << "Uniform [0, 1): " << urd(eng);
If you have multiple sources of randomness you can actually do this (e.g. 2)
std::seed_seq seed{ r1(), r2() };
default_random_engine eng{seed};
uniform_real_distribution<double> urd(0, 1);
cout << "Uniform [0, 1): " << urd(eng);
where r1 , r2 are different random devices , e.g. a thermal noise or quantum source .
Ofcourse you could mix and match
std::seed_seq seed{ r1(), static_cast<long unsigned int>(time(0)) };
default_random_engine eng{seed};
uniform_real_distribution<double> urd(0, 1);
cout << "Uniform [0, 1): " << urd(eng);
Finally, I like to initialize with an one liner:
auto rand = std::bind(std::uniform_real_distribution<double>{0,1},
std::default_random_engine{std::random_device()()});
std::cout << "Uniform [0,1): " << rand();
If you worry about the time(0) having second precision you can overcome this by playing with the high_resolution_clock either by requesting the time since epoch as designated firstly by bames23 below:
static_cast<long unsigned int>(std::chrono::high_resolution_clock::now().time_since_epoch().count())
or maybe just play with CPU randomness
long unsigned int getseed(int const K)
{
typedef std::chrono::high_resolution_clock hiclock;
auto gett= [](std::chrono::time_point<hiclock> t0)
{
auto tn = hiclock::now();
return static_cast<long unsigned int>(std::chrono::duration_cast<std::chrono::microseconds>(tn-t0).count());
};
long unsigned int diffs[10];
diffs[0] = gett(hiclock::now());
for(int i=1; i!=10; i++)
{
auto last = hiclock::now();
for(int k=K; k!=0; k--)
{
diffs[i]= gett(last);
}
}
return *std::max_element(&diffs[1],&diffs[9]);
}