std::random_device
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 | unsigned int |
Member functions
Construction |
|
| constructs the engine (public member function) |
|
| operator=
(deleted) | the assignment operator is deleted (public member function) |
Generation |
|
| advances the engine's state and returns the generated value (public member function) |
|
Characteristics |
|
| 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 is MinGW (bug 338), although replacement implementations exist, such as mingw-std-random_device.
Example
#include <iostream>
#include <string>
#include <map>
#include <random>
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 p : hist) {
std::cout << p.first << " : " << std::string(p.second/100, '*') << '\n';
}
}Possible output:
0 : ******************** 1 : ******************* 2 : ******************** 3 : ******************** 4 : ******************** 5 : ******************* 6 : ******************** 7 : ******************** 8 : ******************* 9 : ********************
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