std::random_device::entropy

Obtains an estimate of the random number device entropy, which is a floating-point value between 0 and log2(max()+1) (which is equal to std::numeric_limits < unsigned int > :: digits ). If the device has n states whose individual probabilities are P0,...,Pn-1, the device entropy S

S = −∑n-1
i=0 Pilog(Pi)

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++ 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++

Example

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

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