random_device(): random_device(/*implementation-defined*/){} | (1) | (since C++11) |
| (2) | (since C++11) |
random_device(const random_device&)= delete; | (3) | (since C++11) |
| | |
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.
[edit]Exceptions
Throws an implementation-defined exception derived fromstd::exception on failure.
The implementation inlibstdc++ expectstoken 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 inlibc++, when configured to use character device as the source, expectstoken to be the name of a character device that produces random numbers when read from; otherwise it expectstoken to be"/dev/urandom".
Both libstdc++ and libc++ throw an exception if provided an unsupported token.Microsoft's stdlib ignores the token entirely.
[edit]Example
Demonstrates commonly available types ofstd::random_device on Linux.
#include <iostream>#include <random> void demo(std::random_device&& rd){staticstd::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
[edit] 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 |
