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 is defined as

S = −∑n-1
i=0 Pilog(Pi)

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

[edit] Return value

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

[edit] 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.

[edit] Example

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

32