std::random_device::random_device (3) Linux Manual Page
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 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: