std::random_device::entropy

double entropy() const noexcept;

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 = −∑n-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 is what std::random_device::entropy() in GNU libstdc++ uses as of version 8.1.

Example

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

32