[rand.device]

If implementation limitations prevent generating nondeterministic random numbers, the implementation may employ a random number engine.

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namespace std { 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);

Throws: A value of an implementation-defined type derived from exception if the random_device cannot be initialized.

Remarks: The semantics of the token parameter and the token value used by the default constructor are implementation-defined.235

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double entropy() const noexcept;

Returns: If the implementation employs a random number engine, returns 0.0.

Otherwise, returns an entropy estimate236 for the random numbers returned by operator(), in the range min() to

log {2 (max() + 1) .}_{2 (max() + 1) .}

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result_type operator()();

Returns: A nondeterministic random value, uniformly distributed between min() and max() (inclusive).

It is implementation-defined how these values are generated.

Throws: A value of an implementation-defined type derived from exception if a random number cannot be obtained.

235)235)

The parameter is intended to allow an implementation to differentiate between different sources of randomness.

236)236)

If a device has n states whose respective probabilities are

{P 0, \dots, {P n - 1, the device entropy S is defined as}_{n - 1, the device entropy S is defined as}}_{0, \dots, {P n - 1, the device entropy S is defined as}_{n - 1, the device entropy S is defined as}}

S = - {\sum n - 1 i = 0 {P i \cdot log {P i .}_{i .}}_{i \cdot log {P i .}_{i .}}}^{n - 1 i = 0 {P i \cdot log {P i .}_{i .}}_{i \cdot log {P i .}_{i .}}}