std::random_device

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Numerics library

Common mathematical functions

Special mathematical functions

Floating-point environment (C++11)

Complex numbers

Numeric arrays

Pseudo-random number generation

Compile-time rational arithmetic (C++11)

Numeric algorithms

gcd (C++17)

lcm (C++17)

Generic numeric operations

iota (C++11)

accumulate

inner_product

adjacent_difference

partial_sum

reduce (C++17)

transform_reduce (C++17)

inclusive_scan (C++17)

exclusive_scan (C++17)

transform_inclusive_scan (C++17)

transform_exclusive_scan (C++17)

Bit operations

bit_cast (C++20)

ispow2 (C++20)

ceil2 (C++20)

floor2 (C++20)

log2p1 (C++20)

Pseudo-random number generation

Uniform random bit generators

UniformRandomBitGenerator (C++20)

Engines and engine adaptors

linear_congruential_engine (C++11)

mersenne_twister_engine (C++11)

subtract_with_carry_engine (C++11)

discard_block_engine (C++11)

independent_bits_engine (C++11)

shuffle_order_engine (C++11)

Non-deterministic generator

random_device (C++11)

Distributions

Uniform distributions

uniform_int_distribution (C++11)

uniform_real_distribution (C++11)

generate_canonical (C++11)

Bernoulli distributions

bernoulli_distribution (C++11)

binomial_distribution (C++11)

negative_binomial_distribution (C++11)

geometric_distribution (C++11)

Poisson distributions

poisson_distribution (C++11)

exponential_distribution (C++11)

gamma_distribution (C++11)

weibull_distribution (C++11)

extreme_value_distribution (C++11)

Normal distributions

normal_distribution (C++11)

lognormal_distribution (C++11)

chi_squared_distribution (C++11)

cauchy_distribution (C++11)

fisher_f_distribution (C++11)

student_t_distribution (C++11)

Sampling distributions

discrete_distribution (C++11)

piecewise_constant_distribution (C++11)

piecewise_linear_distribution (C++11)

Seed Sequences

seed_seq (C++11)

C library

rand

srand

RAND_MAX

std::random_device

Member functions

random_device::random_device

Generation

random_device::operator()

Characteristics

random_device::entropy

random_device::min

random_device::max

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

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