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Showing content from https://cplusplus.com/reference/random/random_device/entropy/ below:

public member function

std::random_device::entropy

double entropy() const noexcept;

Return entropy

Returns an entropy estimate for the random numbers returned by operator().

The value is expressed on a base-2 scale, with a value between zero and log2(max()+1).

If the library implementation employs a random number engine instead of a real-random number generator, the value returned by this function is always zero.

Parameters none

Return value The entropy estimate for the random numbers generated.

Example

// random_device example
#include <iostream>
#include <random>

int main ()
{
  std::random_device rd;

  std::cout << "default random_device characteristics:" << std::endl;
  std::cout << "minimum: " << rd.min() << std::endl;
  std::cout << "maximum: " << rd.max() << std::endl;
  std::cout << "entropy: " << rd.entropy() << std::endl;
  std::cout << "a random number: " << rd() << std::endl;

  return 0;
}

Possible output:

default random_device characteristics:
minimum: 0
maximum: 4294967295
entropy: 32
a random number: 2755249813