std::random_device
From cppreference.com
| 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.
Member types
| Member type | Definition |
result_type (C++11)
|
unsigned int
|
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) |
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.
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 : ********************