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 exception 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", "hw", "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 commonly available types of std::random_device on Linux.

// Run this code

#include <iostream>
#include <random>

void demo(std::random_device&& rd)
{
static std::uniform_int_distribution<int> d(0, 9);
for (int n = 0; n != 10; ++n)
std::cout << d(rd) << ' ';
std::cout << '\n';
}

int main()
{
// Note: How the supplied token is handled is implementation-defined!

// Default token for random_device is usually /dev/urandom on Linux
demo(std::random_device {});

// Request /dev/random, blocks when entropy is empty
// Works on libstdc++, ignored in msvc++, might throw on libc++ (as of Nov 2022)
demo(std::random_device {"/dev/random"});

// Request non-blocking /dev/urandom, ensures that RDRAND is not used
// Works on libstdc++ and libc++, ignored in msvc++ (as of Nov 2022)
demo(std::random_device {"/dev/urandom"});

// Request "hw", will use hardware-based random generation like rdrand
// Works on libstdc++, ignored in msvc++, throws on libc++ (as of Nov 2022)
demo(std::random_device {"hw"});
}

Possible output:¶

9 5 2 7 5 9 4 1 0 7
4 7 6 5 1 5 5 1 8 6
3 3 6 1 4 1 4 1 0 2
4 6 3 9 1 9 4 0 9 3

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