# std::random_device

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< cpp‎ | numeric‎ | random

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
(constructor) (C++11) | constructs the engine (public member function)
operator= (deleted) (C++11) | the assignment operator is deleted (public member function)
##### Generation
operator() (C++11) | advances the engine's state and returns the generated value (public member function)
##### Characteristics
entropy (C++11) | obtains the entropy estimate for the non-deterministic random number generator (public member function)
min [static] (C++11) | gets the smallest possible value in the output range (public static member function)
max [static] (C++11) | 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 (bug 338, fixed since GCC 9.2). The latest MinGW 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 : ********************

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