std::random_device(3)

NAME
std::random_device - std::random_device

Synopsis
   Defined in header <random>
   class random_device; (since C++11)
   std::random_device is a uniformly-distributed integer random number generator that
   produces non-deterministic random numbers.
   std::random_device may be implemented in terms of an implementation-defined
   pseudo-random number engine if a non-deterministic source (e.g. a hardware device)
   is not available to the implementation. In this case each std::random_device object
   may generate the same number sequence.

Member types
   Member type Definition
   result_type unsigned int

Member functions

Construction
   constructor constructs the engine
   (C++11) (public member function)
   operator= the assignment operator is deleted
   (deleted) (public member function)

Generation
   operator() advances the engine's state and returns the generated value
   (C++11) (public member function)

Characteristics
                    obtains the entropy estimate for the non-deterministic random
   entropy number generator
                    (public member function)
   min gets the smallest possible value in the output range
   [static] (C++11) (public static member function)
   max gets the largest possible value in the output range
   [static] (C++11) (public static member function)

Notes
   A notable implementation where std::random_device is deterministic is old versions
   of MinGW (bug 338, fixed since GCC 9.2). The latest MinGW versions can be downloaded
   from GCC with the MCF thread model.

Example
// Run this code
 #include <iostream>
 #include <string>
 #include <map>
 #include <random>
 int main()
 {
     std::random_device rd;
     std::map<int, int> hist;
     std::uniform_int_distribution<int> dist(0, 9);
     for (int n = 0; n < 20000; ++n) {
         ++hist[dist(rd)]; // note: demo only: the performance of many
                        // implementations of random_device degrades sharply
                        // once the entropy pool is exhausted. For practical use
                        // random_device is generally only used to seed
                        // a PRNG such as mt19937
     }
     for (auto p : hist) {
         std::cout << p.first << " : " << std::string(p.second/100, '*') << '\n';
     }
 }

Possible output:
 0 : ********************
 1 : *******************
 2 : ********************
 3 : ********************
 4 : ********************
 5 : *******************
 6 : ********************
 7 : ********************
 8 : *******************
 9 : ********************