std::random_device::random_device

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

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Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

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Technical specifications

Symbols index

External libraries

 

Numerics library

Common mathematical functions

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Mathematical constants (C++20)

Floating-point environment (C++11)

Complex numbers

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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)

Bit operations

bit_cast (C++20)

byteswap (C++23)

has_single_bit (C++20)

bit_ceil (C++20)

bit_floor (C++20)

bit_width (C++20)

rotl (C++20)

rotr (C++20)

countl_zero (C++20)

countl_one (C++20)

countr_zero (C++20)

countr_one (C++20)

popcount (C++20)

endian (C++20)

 

Pseudo-random number generation

Uniform random bit generators

uniform_random_bit_generator (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 (C++11)

Generation

random_device::operator() (C++11)

Characteristics

random_device::entropy (C++11)

random_device::min (C++11)

random_device::max (C++11)

 

random_device() : random_device(/*implementation-defined*/) {}	(1)	(since C++11)
explicit random_device(const std::string& token);	(2)	(since C++11)
random_device(const random_device& ) = delete;	(3)	(since C++11)

1) Default constructs a new std::random_device object with an implementation-defined token.

2) Constructs a new std::random_device object, making use of the argument token in an implementation-defined manner.

3) The copy constructor is deleted: std::random_device is not copyable nor movable.

Exceptions

Throws an implementation-defined exceptions derived from std::exception on failure.

Notes

The implementation in libstdc++ expects token to name the source of random bytes. Possible token values include "default", "rand_s", "rdseed", "rdrand", "rdrnd", "/dev/urandom", "/dev/random", "mt19937", and integer string specifying the seed of the mt19937 engine. (Token values other than "default" are only valid for certain targets.)

The implementation in libc++, when configured to use character device as the source, expects token to be the name of a character device that produces random numbers when read from; otherwise it expects token to be "/dev/urandom".

Both libstdc++ and libc++ throw an exception if provided an unsupported token. Microsoft's stdlib ignores the token entirely.

Example

Demonstrates the two commonly available types of std::random_device on Linux

Run this code

#include <iostream>
#include <random>
 
int main()
{
    std::uniform_int_distribution<int> d(0, 10);
 
    std::random_device rd1; // uses RDRND or /dev/urandom
    for(int n = 0; n < 10; ++n)
        std::cout << d(rd1) << ' ';
    std::cout << '\n';
 
    std::random_device rd2("/dev/random"); // much slower on Linux
    for(int n = 0; n < 10; ++n)
        std::cout << d(rd2) << ' ';
    std::cout << '\n';
}

Possible output:

7 10 7 0 4 4 6 9 4 7 
2 4 10 6 3 2 0 6 3 7

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P0935R0	C++11	default constructor was explicit	made implicit