std::random_device::entropy

 

C++

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std::random_device

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random_device::operator()

Characteristics

random_device::entropy

random_device::min

random_device::max

 

double entropy() const;		(since C++11)

Obtains an estimate of the random number device entropy, which is a floating-point value between 0 and log
2(max()+1) (which is equal to std::numeric_limits<unsigned int>::digits). If the device has n states whose individual probabilities are P
0,...,P
n-1, the device entropy S is defined as

S = -Σn-1
i=0P
ilog(P
i)

A deterministic random number generator (e.g. a pseudo-random engine) has entropy zero.

Exceptions

noexcept specification:  

noexcept

  

Return value

The value of the device entropy, or zero if not applicable.

Notes

This function is not fully implemented in some standard libraries. For example, GNU libstdc++ and LLVM libc++ always return zero even though the device is non-deterministic (for libstdc++, see bug 67578). In comparison, Microsoft Visual C++ implementation always returns 32, and boost.random returns 10.

Example

Example output on one of the implementations

Run this code

#include <iostream>
#include <random>
 
int main()
{
    std::random_device rd;
    std::cout << rd.entropy() << '\n';
}

Possible output:

32