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=0P

ilog(P

i).

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

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

This function is not fully implemented in some standard libraries. For example, LLVM libc++ 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 output on one of the implementations.

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

Possible output:

32

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