std::random_device

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Numerics library

Common mathematical functions
Mathematical special functions(C++17)
Mathematical constants(C++20)
Basic linear algebra algorithms(C++26)
Data-parallel types (SIMD)(C++26)
Floating-point environment(C++11)
Complex numbers
Numeric array (valarray)
Pseudo-random number generation
Bit manipulation(C++20)
Saturation arithmetic(C++26)
Factor operations
gcd (C++17)	lcm (C++17)
Interpolations
midpoint (C++20)	lerp (C++20)
Generic numeric operations
iota (C++11) ranges::iota (C++23) 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)
C-style checked integer arithmetic
ckd_add (C++26) ckd_sub (C++26)	ckd_mul (C++26)

 

Pseudo-random number generation

Uniform random bit generators   uniform_random_bit_generator (C++20) Random number engines linear_congruential_engine (C++11) mersenne_twister_engine (C++11) subtract_with_carry_engine (C++11) philox_engine (C++26) Random number engine adaptors discard_block_engine (C++11) independent_bits_engine (C++11) shuffle_order_engine (C++11) Predefined random number generators Non-deterministic random numbers random_device (C++11) Utilities generate_canonical (C++11) seed_seq (C++11) Random number algorithms ranges::generate_random (C++26) C random library rand    srand    RAND_MAX

Random number 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)

 

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(C++11)	unsignedint

Member functions

Construction
(constructor)	constructs the engine (public member function)
operator= (deleted)(C++11)	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 (C++11)	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 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 : ********************