std::random_device::random_device (3) - Linux Manuals

std::random_device::random_device: std::random_device::random_device

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 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