24 Numerics library [numerics]

24.7 Random number generation [rand]

24.7.6 Class random_­device [rand.device]

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A random_­device uniform random bit generator produces nondeterministic random numbers.
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If implementation limitations prevent generating nondeterministic random numbers, the implementation may employ a random number engine.
class random_device {
public:
  // types
  using result_type = unsigned int;

  // generator characteristics
  static constexpr result_type min() { return numeric_limits<result_type>::min(); }
  static constexpr result_type max() { return numeric_limits<result_type>::max(); }

  // constructors
  random_device() : random_device(implementation-defined) {}
  explicit random_device(const string& token);

  // generating functions
  result_type operator()();

  // property functions
  double entropy() const noexcept;

  // no copy functions
  random_device(const random_device& ) = delete;
  void operator=(const random_device& ) = delete;
};
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explicit random_device(const string& token);
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Effects: Constructs a random_­device nondeterministic uniform random bit generator object.
The semantics of the token parameter and the token value used by the default constructor are implementation-defined.250
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Throws: A value of an implementation-defined type derived from exception if the random_­device could not be initialized.
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double entropy() const noexcept;
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Returns: If the implementation employs a random number engine, returns 0.0.
Otherwise, returns an entropy estimate251 for the random numbers returned by operator(), in the range min() to .
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result_type operator()();
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Returns: A nondeterministic random value, uniformly distributed between min() and max(), inclusive.
It is implementation-defined how these values are generated.
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Throws: A value of an implementation-defined type derived from exception if a random number could not be obtained.
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The parameter is intended to allow an implementation to differentiate between different sources of randomness.
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If a device has n states whose respective probabilities are , the device entropy S is defined as
.