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std::random_device::random_device

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.

#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

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