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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