std::random...andom_device(3)

okay so bestie, you opened std::random...andom_device(3) — the C++ Standard Libary manual page for std::random_device::random_device (yes, “Libary” is literally how it’s spelled up top, we’re keeping the typo 💅). This page is short but it has opinions about tokens, and we’re getting into all of them.

💖 NAME 💖

std::random_device::random_device — std::random_device::random_device

that’s the whole NAME line, iconic repetition, no notes ✨

🌸 Synopsis 🌸

here’s the overload lineup, C++11 energy and all 🎀

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. (1) — Default constructs a new std::random_device object with an implementation-defined token.
2. (2) — Constructs a new std::random_device object, making use of the argument token in an implementation-defined manner.
3. (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. so if it’s gonna fail, it fails with mystery implementation-defined flavor 😭

🫶 Notes 🫶

okay, token tea time ☕ — and bestie, the standard is not giving us one universal meaning for token. it’s giving implementation-defined.

• 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 an 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. token chaos, truly 🌈

🦋 Example 🦋

Demonstrates commonly available types of std::random_device on Linux. The original page has a little // Run this code marker before the block, so here’s that whole vibe 👇

#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: P0935R0 · Applied to: C++11 · Behavior as published: default constructor was explicit · Correct behavior: made implicit

mwah 💋 this page came from http://cppreference.com, dated 2024.06.10, and the footer page name is std::random...andom_device(3) — stay random, bestie 🎀✨