random_device Class

Generates a random sequence from an external device.


class random_device {
   typedef unsigned int result_type;

   // constructor
   explicit random_device(const std::string& token = "");

   // properties
   static result_type min();
   static result_type max();
   double entropy() const;

   // generate
   result_type operator()();

   // no-copy functions
   random_device(const random_device&) = delete;
   void operator=(const random_device&) = delete;




The class describes a source of random numbers, and is allowed but not required to be non-deterministic or cryptographically secure by the ISO C++ Standard. In the Visual Studio implementation the values produced are non-deterministic and cryptographically secure, but runs more slowly than generators created from engines and engine adaptors (such as mersenne_twister_engine, the high quality and fast engine of choice for most applications).

random_device results are uniformly distributed in the closed range [ 0, 232).

random_device is not guaranteed to result in a non-blocking call.

Generally, random_device is used to seed other generators created with engines or engine adaptors. For more information, see <random>.


The following code demonstrates basic functionality of this class and example results. Because of the non-deterministic nature of random_device, the random values shown in the Output section will not match your results. This is normal and expected.

// random_device_engine.cpp
// cl.exe /W4 /nologo /EHsc /MTd
#include <random>
#include <iostream>
using namespace std;

int main()
    random_device gen;

    cout << "entropy == " << gen.entropy() << endl;
    cout << "min == " << gen.min() << endl;
    cout << "max == " << gen.max() << endl;

    cout << "a random value == " << gen() << endl;
    cout << "a random value == " << gen() << endl;
    cout << "a random value == " << gen() << endl;
entropy == 32
min == 0
max == 4294967295
a random value == 2378414971
a random value == 3633694716
a random value == 213725214

This example is simplistic and not representative of the general use-case for this generator. For a more representative code example, see <random>.


Header: <random>

Namespace: std


Constructs the generator.

random_device(const std::string& = "");


The constructor initializes the generator as needed, ignoring the string parameter. Throws a value of an implementation-defined type derived from exception if the random_device could not be initialized.


Estimates the randomness of the source.

double entropy() const noexcept;


The member function returns an estimate of the randomness of the source, as measured in bits.


Returns a random value.

result_type operator()();


Returns values uniformly distributed in the closed interval [ min, max] as determined by member functions min() and max(). Throws a value of an implementation-defined type derived from exception if a random number could not be obtained.

See also