NAME

std::random_device::entropy - std::random_device::entropy

Synopsis

double entropy() const noexcept; (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=0 P
ilog(P
i)

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

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, LLVM
libc++ prior to version 12 always returns zero even though the device is
non-deterministic. In comparison, Microsoft Visual C++ implementation always returns
32, and boost.random returns 10.

The entropy of the Linux kernel device /dev/urandom may be obtained using ioctl
RNDGETENTCNT - that's what std::random_device::entropy() in GNU libstdc++ uses as of
version 8.1

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