# std::random_device::random_device

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< cpp‎ | numeric‎ | random‎ | random device

random_device() : random_device(/*implementation-defined*/) {} | (1) | (since C++11)
explicit random_device(const std::string & token); | (2) | (since C++11)
random_device(const random_device& ) = delete; | (3) | (since C++11)

1) Default constructs a new std::random_device object with an implementation-defined `token`.
2) Constructs a new std::random_device object, making use of the argument `token` in an implementation-defined manner.
3) The copy constructor is deleted: `std::random_device` is not copyable nor movable.

### Exceptions

Throws an implementation-defined exceptions derived from std::exception on failure.

### Notes

The implementations in libc++ and libstdc++ expect `token` to be the name of a character device that produces random numbers when read from, with the default value "/dev/urandom", although where the CPU instruction RDRND is available, libstdc++ uses that as the default.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P0935R0 | C++11 | default constructor was explicit | made implicit

### Example

Demonstrates the two commonly available types of `std::random_device` on Linux

Run this code

    #include <iostream>
    #include <random>

    int main()
    {

        std::uniform_int_distribution<int> d(0, 10);

        std::random_device rd1; // uses RDRND or /dev/urandom
        for(int n = 0; n < 10; ++n)
            std::cout << d(rd1) << ' ';
        std::cout << '\n';

        std::random_device rd2("/dev/random"); // much slower on Linux
        for(int n = 0; n < 10; ++n)
            std::cout << d(rd2) << ' ';
        std::cout << '\n';
    }

Possible output:

    7 10 7 0 4 4 6 9 4 7
    2 4 10 6 3 2 0 6 3 7

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