std::random_device
From cppreference.com
C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Numerics library
Mathematical special functions (C++17)
Mathematical constants (C++20)
Basic linear algebra algorithms (C++26)
Data-parallel types (SIMD) (C++26)
Floating-point environment (C++11)
Bit manipulation (C++20)
Saturation arithmetic (C++26)
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
(C++17)
Pseudo-random number generation
(C++20)
(C++11)
(C++11)
(C++11)
(C++26)
(C++11)
(C++11)
(C++11)
random_device
(C++11)
(C++11)
(C++11)
(C++26)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
std::random_device
Member functions
Generation
Characteristics
Defined in header
<random>
class random_device;
(since C++11)
std::random_device is a uniformly-distributed integer random number generator that produces non-deterministic random numbers.
std::random_device may be implemented in terms of an implementation-defined pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation. In this case each std::random_device object may generate the same number sequence.
Contents
[edit] Member types
Member type
Definition
result_type (C++11)
unsigned int
[edit] Member functions
Construction
operator=
(deleted) (C++11)
(public member function)
Generation
Characteristics
(C++11)
(public member function) [edit]
[static]
(public static member function) [edit]
[edit] Notes
A notable implementation where std::random_device is deterministic in old versions of MinGW-w64 (bug 338, fixed since GCC 9.2). The latest MinGW-w64 versions can be downloaded from GCC with the MCF thread model.
[edit] Example
Run this code
#include <iostream> #include <map> #include <random> #include <string> int main() { std::random_device rd; std::map <int, int> hist; std::uniform_int_distribution <int> dist(0, 9); for (int n = 0; n != 20000; ++n) ++hist[dist(rd)]; // note: demo only: the performance of many // implementations of random_device degrades sharply // once the entropy pool is exhausted. For practical use // random_device is generally only used to seed // a PRNG such as mt19937 for (auto [x, y] : hist) std::cout << x << " : " << std::string (y / 100, '*') << '\n'; }
Possible output:
0 : ******************** 1 : ******************* 2 : ******************** 3 : ******************** 4 : ******************** 5 : ******************* 6 : ******************** 7 : ******************** 8 : ******************* 9 : ********************