std::random_device::random_device

 

C++

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Technical Specifications

 

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Non-deterministic generator

random_device (C++11)

Distributions

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uniform_int_distribution (C++11)

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poisson_distribution (C++11)

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normal_distribution (C++11)

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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

 

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 implementations in libc++ and libstdc++ expect token to be the name of a character device that produces random numbers when read from, with the default value "/dev/urandom", although where the CPU instruction RDRND is available, libstdc++ uses that as the default.

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

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