std:: random_device

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std::random_device
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 (C++11) unsigned int

Member functions

Construction
constructs the engine
(public member function)
operator=
(deleted) (C++11)
the assignment operator is deleted
(public member function)
Generation
advances the engine's state and returns the generated value
(public member function)
Characteristics
(C++11)
obtains the entropy estimate for the non-deterministic random number generator
(public member function)
[static]
gets the smallest possible value in the output range
(public static member function)
[static]
gets the largest possible value in the output range
(public static member function)

Notes

A notable implementation where std::random_device is deterministic in old versions of MinGW-w64 ( bug 338 , fixed since GCC 9.2). The latest MinGW-w64 versions can be downloaded from GCC with the MCF thread model .

Example

Run this code 
#include <iostream>
#include <map>
#include <random>
#include <string>
 
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 [x, y] : hist)
        std::cout << x << " : " << std::string(y / 100, '*') << '\n';
}

Possible output: 

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