< cpp‎ | numeric‎ | random
Pseudo-random number generation
Uniform random bit generators
Engines and engine adaptors
Non-deterministic generator
Uniform distributions
Bernoulli distributions
Poisson distributions
Normal distributions
Sampling distributions
Seed Sequences
C library
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.


[edit] Member types

Member type Definition
result_type unsigned int

[edit] Member functions

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

[edit] Notes

A notable implementation where std::random_device is deterministic is old versions of MinGW (bug 338, fixed since GCC 9.2), although replacement implementations exist, such as mingw-std-random_device. The latest MinGW Versions can be downloaded from GCC with the MCF thread model.

[edit] Example

#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 : ********************