C++11: How to set seed using <random>

Question

I am exercising the random library, new to C++11. I wrote the following minimal program:

#include <iostream>
#include <random>
using namespace std;
int main() {
    default_random_engine eng;
    uniform_real_distribution<double> urd(0, 1);
    cout << "Uniform [0, 1): " << urd(eng);
}

When I run this repeatedly it gives the same output each time:

>a
Uniform [0, 1): 0.131538
>a
Uniform [0, 1): 0.131538
>a
Uniform [0, 1): 0.131538

I would like to have the program set the seed differently each time it is called, so that a different random number is generated each time. I am aware that random provides a facility called seed_seq, but I find the explanation of it (at cplusplus.com) totally obscure:

http://www.cplusplus.com/reference/random/seed_seq/

I'd appreciate advice on how to have a program generate a new seed each time it is called: The simpler the better.

My platform(s):

• Windows 7 : TDM-GCC compiler

Answer 1

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]);
}

Answer 2

#include <iostream>
#include <random>

using namespace std;

int main() {
    std::random_device r;                                       // 1
    std::seed_seq seed{r(), r(), r(), r(), r(), r(), r(), r()}; // 2
    std::mt19937 eng(seed);                                     // 3

    uniform_real_distribution<double> urd(0, 1);

    cout << "Uniform [0, 1): " << urd(eng);
}

In order to get unpredictable results from a pseudo-random number generator we need a source of unpredictable seed data. On 1 we create a std::random_device for this purpose. On 2 we use a std::seed_seq to combine several values produced by random_device into a form suitable for seeding a pseudo-random number generator. The more unpredictable data that is fed into the seed_seq, the less predictable the results of the seeded engine will be. On 3 we create a random number engine using the seed_seq to seed the engine's initial state.

A seed_seq can be used to initialize multiple random number engines; seed_seq will produce the same seed data each time it is used.

Note: Not all implemenations provide a source of non-deterministic data. Check your implementation's documentation for std::random_device.

---

If your platform does not provide a non-deterministic random_device then some other sources can be used for seeding. The article Simple Portable C++ Seed Entropy suggests a number of alternative sources:

• A high resolution clock such as std::chrono::high_resolution_clock (time() typically has a resolution of one second which generally too low)
• Memory configuration which on modern OSs varies due to address space layout randomization (ASLR)
• CPU counters or random number generators. C++ does not provide standardized access to these so I won't use them.
• thread id
• A simple counter (which only matters if you seed more than once)

For example:

#include <chrono>
#include <iostream>
#include <random>
#include <thread>
#include <utility>

using namespace std;

// we only use the address of this function
static void seed_function() {}

int main() {
    // Variables used in seeding
    static long long seed_counter = 0;
    int var;
    void *x = std::malloc(sizeof(int));
    free(x);

    std::seed_seq seed{
        // Time
        static_cast<long long>(std::chrono::high_resolution_clock::now()
                                   .time_since_epoch()
                                   .count()),
        // ASLR
        static_cast<long long>(reinterpret_cast<intptr_t>(&seed_counter)),
        static_cast<long long>(reinterpret_cast<intptr_t>(&var)),
        static_cast<long long>(reinterpret_cast<intptr_t>(x)),
        static_cast<long long>(reinterpret_cast<intptr_t>(&seed_function)),
        static_cast<long long>(reinterpret_cast<intptr_t>(&_Exit)),
        // Thread id
        static_cast<long long>(
            std::hash<std::thread::id>()(std::this_thread::get_id())),
        // counter
        ++seed_counter};

    std::mt19937 eng(seed);

    uniform_real_distribution<double> urd(0, 1);

    cout << "Uniform [0, 1): " << urd(eng);
}