A random_device uniform random bit generator produces nondeterministic random numbers.
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 explicit random_device(const string& token = implementation-defined); // 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; };
explicit random_device(const string& token = implementation-defined);
Effects: Constructs a random_device nondeterministic uniform random bit generator object. The semantics and default value of the token parameter are implementation-defined. 272
double entropy() const noexcept;
Returns: If the implementation employs a random number engine, returns 0.0. Otherwise, returns an entropy estimate273 for the random numbers returned by operator(), in the range min() to log2( max()+1).
The parameter is intended to allow an implementation to differentiate between different sources of randomness.
If a device has n states
whose respective probabilities are
P0, …, Pn-1 ,
the device entropy S is defined as
.