NAME¶

std::random_device::random_device - std::random_device::random_device

Synopsis¶

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