Defined in header <random>  

class random_device;  (since C++11) 
std::random_device
is a uniformlydistributed integer random number generator that produces nondeterministic random numbers.
std::random_device
may be implemented in terms of an implementationdefined pseudorandom number engine if a nondeterministic 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 type  Definition 

result_type  unsigned int 
Construction 

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

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

obtains the entropy estimate for the nondeterministic random number generator (public member function) 

[static] (C++11)  gets the smallest possible value in the output range (public static member function) 
[static] (C++11)  gets the largest possible value in the output range (public static member function) 
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.
#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 : ********************
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