double entropy() const;
Obtains an estimate of the random number device entropy, which is a floating-point value between min() and log
2(max()+1). If the device has n states whose individual probabilities are P
n-1, the device probability 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, gcc and clang always return zero even though the device is non-deterministic. In comparison, Visual C++ always returns 32, and boost.random returns 10.