std::random_device - cppreference.com (original) (raw)
| | | | | --------------------- | | ------------- | | 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 | |
|---|---|
| (constructor) | constructs the engine (public member function) [edit] |
| operator=(deleted) (C++11) | the assignment operator is deleted (public member function) |
| Generation | |
| operator() | advances the engine's state and returns the generated value (public member function) [edit] |
| Characteristics | |
| entropy(C++11) | obtains the entropy estimate for the non-deterministic random number generator (public member function) [edit] |
| min[static] | gets the smallest possible value in the output range (public static member function) [edit] |
| max[static] | gets the largest possible value in the output range (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
#include #include #include #include int main() { std::random_device rd; std::map<int, int> hist; std::uniform_int_distribution 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 : ********************