Difference between revisions of "cpp/numeric/random/random device"
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===Notes=== | ===Notes=== | ||
| − | A notable implementation where {{ttt|std::random_device}} is deterministic | + | A notable implementation where {{ttt|std::random_device}} is deterministic in old versions of MinGW-w64 ([https://sourceforge.net/p/mingw-w64/bugs/338/ bug 338], fixed since GCC 9.2). The latest MinGW-w64 versions can be downloaded from [https://gcc-mcf.lhmouse.com/ GCC with the MCF thread model]. |
===Example=== | ===Example=== | ||
Latest revision as of 21:34, 20 November 2023
| Defined in header <random>
|
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| 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 | |
| constructs the engine (public member function) | |
| operator= (deleted) (C++11) |
the assignment operator is deleted (public member function) |
Generation | |
| advances the engine's state and returns the generated value (public member function) | |
Characteristics | |
| (C++11) |
obtains the entropy estimate for the non-deterministic random number generator (public member function) |
| [static] |
gets the smallest possible value in the output range (public static member function) |
| [static] |
gets the largest possible value in the output range (public static member function) |
[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 : ********************
