double entropy() const;
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
n-1, the device entropy S is defined as
S = -Σn-1
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.
This function is not fully implemented in some standard libraries. For example, GNU libstdc++ and LLVM libc++ always return zero even though the device is non-deterministic (for libstdc++, see bug 67578). 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