how to make each thread use its own RNG in C++11

Question

I\'m using the new random number generators in in C++11. Although there are varying opinions, from this thread it seems that the majority believe they are not thread safe. As a

Answer 1

You must no share instances of random engine between multiple threads. You should either lock a single engine or create one engine for each thread (with different seed (please note the answer of e4e5f4 regarding creation of parallel MT engines)). In case of OpenMP you can easily store one engine per thread in a vector and retrieve it by result of omp_get_thread_num() which lies between 0 and omp_get_num_threads()–1.

class RNG
{
public:
    typedef std::mt19937 Engine;
    typedef std::uniform_real_distribution Distribution;

    RNG() : engines(), distribution(0.0, 1.0)
    {
        int threads = std::max(1, omp_get_max_threads());
        for(int seed = 0; seed < threads; ++seed)
        {
            engines.push_back(Engine(seed));
        }
    }

    double operator()()
    {
        int id = omp_get_thread_num();
        return distribution(engines[id]);
    }

    std::vector engines;
    Distribution distribution;
};

int main()
{
     RNG rand;
     unsigned long app = 0;

     #pragma omp parallel for reduction(+:app)
     for (unsigned long long i = 0; i < 2000000000; i++)
     {
         if(rand() < 0.5) app++;
     }
}