std::random_device

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< cpplrm; | numericlrm; | random
Pseudo-random number generation
Uniform random bit generators
Engines and engine adaptors
Non-deterministic generator
random_device
(C++11)
Distributions
Uniform distributions
Bernoulli distributions
Poisson distributions
Normal distributions
Sampling distributions
Seed Sequences
(C++11)
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.

Member types

Member type Definition
result_type unsigned int

Member functions

Construction
constructs the engine
(public member function)
operator=
(deleted)
the assignment operator is deleted
(public member function)
Generation
advances the engine's state and returns the generated value
(public member function)
Characteristics
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 is MinGW (bug 338), although replacement implementations exist, such as mingw-std-random_device.

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: ********************