std::random_device::operator()

// 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;
}