Obtains an estimate of the random number device entropy, which is a floating-point value between0 andlog2(max()+1) (which is equal tostd::numeric_limits<unsignedint>::digits). If the device hasn states whose individual probabilities areP0,...,Pn-1, the device entropyS 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 returns32, andboost.random returns10.

The entropy of the Linux kernel device/dev/urandom may be obtained usingioctl RNDGETENTCNT — that is whatstd::random_device::entropy() inGNU 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

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