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