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std::random...andom_device(3) C++ Standard Libarystd::random...andom_device(3) 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 imple- mentation-defined token. 2) Constructs a new std::random_device object, making use of the ar- gument token in an implementation-defined manner. 3) The copy constructor is deleted: std::random_device is not copy- able nor movable. Exceptions Throws an implementation-defined exceptions derived from std::excep- tion on failure. Notes The implementation in libstdc++ expects token to name the source of random bytes. Possible token values include "default", "rand_s", "rdseed", "rdrand", "rdrnd", "/dev/urandom", "/dev/random", "mt19937", and integer string speci- fying 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 de- vice 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 unsup- ported token. Microsoft's stdlib ignores the token entirely. 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 Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior P0935R0 C++11 default constructor was explicit made implicit http://cppreference.com 2022.07.31 std::random...andom_device(3)
NAME | Synopsis | Exceptions | Notes | Example | Possible output:
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