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## Class random_device

boost::random::random_device

## Synopsis

    // In header: <boost/random/random_device.hpp>

    class random_device {
    public:
      // types
      typedef unsigned int result_type;

      // construct/copy/destruct
      random_device();
      explicit random_device(const std::string &);
      ~random_device();

      // public static functions
      static result_type min();
      static result_type max();

      // public member functions
      double entropy() const;
      unsigned int operator()();
      template<typename Iter> void generate(Iter, Iter);

      // public data members
      static const bool has_fixed_range;
    };

## Description

Class random_device models a non-deterministic random number generator . It uses one or more implementation-defined stochastic processes to generate a sequence of uniformly distributed non-deterministic random numbers. For those environments where a non-deterministic random number generator is not available, class random_device must not be implemented. See

> "Randomness Recommendations for Security", D. Eastlake, S. Crocker, J. Schiller, Network Working Group, RFC 1750, December 1994

for further discussions.

| Note
Some operating systems abstract the computer hardware enough to make it difficult to non-intrusively monitor stochastic processes. However, several do provide a special device for exactly this purpose. It seems to be impossible to emulate the functionality using Standard C++ only, so users should be aware that this class may not be available on all platforms.

Implementation Note for Linux

On the Linux operating system, token is interpreted as a filesystem path. It is assumed that this path denotes an operating system pseudo-device which generates a stream of non-deterministic random numbers. The pseudo-device should never signal an error or end-of-file. Otherwise, `std::ios_base::failure` is thrown. By default, random_device uses the /dev/urandom pseudo-device to retrieve the random numbers. Another option would be to specify the /dev/random pseudo-device, which blocks on reads if the entropy pool has no more random bits available.

Implementation Note for Windows

On the Windows operating system, token is interpreted as the name of a cryptographic service provider. By default random_device uses MS_DEF_PROV.

Performance

The test program nondet_random_speed.cpp measures the execution times of the random_device.hpp implementation of the above algorithms in a tight loop. The performance has been evaluated on an Intel(R) Core(TM) i7 CPU Q 840 @ 1.87GHz, 1867 Mhz with Visual C++ 2010, Microsoft Windows 7 Professional and with gcc 4.4.5, Ubuntu Linux 2.6.35-25-generic.

Platform | time per invocation [microseconds]
Windows | 2.9
Linux | 1.7

The measurement error is estimated at +/- 1 usec.

### `random_device` public construct/copy/destruct

1.     random_device();

   Constructs a ``random_device``, optionally using the default device.

2.     explicit random_device(const std::string & token);

   Constructs a ``random_device``, optionally using the given token as an access specification (for example, a URL) to some implementation-defined service for monitoring a stochastic process.

3.     ~random_device();

### `random_device` public static functions

1.     static result_type min();

   Returns the smallest value that the `random_device` can produce.

2.     static result_type max();

   Returns the largest value that the `random_device` can produce.

### `random_device` public member functions

1.     double entropy() const;

   Returns: An entropy estimate for the random numbers returned by operator(), in the range min() to log2( max()+1). A deterministic random number generator (e.g. a pseudo-random number engine) has entropy 0.

   Throws: Nothing.

2.     unsigned int operator()();

   Returns a random value in the range [min, max].

3.     template<typename Iter> void generate(Iter begin, Iter end);

   Fills a range with random 32-bit values.