[rand.device]

# 29 Numerics library [[numerics]](./#numerics)

## 29.5 Random number generation [[rand]](rand#device)

### 29.5.7 Class random_device [rand.device]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4027)

A random_device uniform random bit generator
produces nondeterministic random numbers[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4032)

If implementation limitations
prevent generating nondeterministic random numbers,
the implementation may employ a random number engine[.](#2.sentence-1)

[🔗](#lib:random_device_)

namespace std {class random_device {public:// typesusing result_type = unsigned int; // generator characteristicsstatic 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 functionsdouble entropy() const noexcept; // no copy functions random_device(const random_device&) = delete; void operator=(const random_device&) = delete; };}

[🔗](#lib:random_device,constructor)

`explicit random_device(const string& token);
`

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4074)

*Throws*: A value of an implementation-defined type
 derived from exception if the random_device cannot be initialized[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4080)

*Remarks*: The semantics of the token parameter
 and the token value used by the default constructor are implementation-defined[.](#4.sentence-1)[243](#footnote-243 "The parameter is intended to allow an implementation to differentiate between different sources of randomness.")

[🔗](#lib:entropy,random_device)

`double entropy() const noexcept;
`

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4098)

*Returns*: If the implementation employs a random number engine,
 returns 0.0[.](#5.sentence-1)

Otherwise, returns an entropy estimate[244](#footnote-244 "If a device has n states whose respective probabilities are �…�, the device entropy S is defined as S = - ∑�.") for the random numbers returned by operator(),
 in the range min() to log2(max()+1)[.](#5.sentence-2)

[🔗](#lib:operator(),random_device)

`result_type operator()();
`

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4123)

*Returns*: A nondeterministic random value,
 uniformly distributed
 between min() and max() (inclusive)[.](#6.sentence-1)

It is implementation-defined
 how these values are generated[.](#6.sentence-2)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L4131)

*Throws*: A value of an implementation-defined
 type derived from exception if a random number cannot be obtained[.](#7.sentence-1)

[243)](#footnote-243)[243)](#footnoteref-243)

The parameter is intended
 to allow an implementation to differentiate
 between different sources of randomness[.](#footnote-243.sentence-1)

[244)](#footnote-244)[244)](#footnoteref-244)

If a device has n states
 whose respective probabilities are P0,…,Pn−1,
 the device entropy S is defined as  

S=−∑n−1i=0Piâ‹logPi[.](#footnote-244.sentence-2)