True random value generator

To generate true random values that can be used for cryptography std::random_device has to be used as generator.

#include <iostream>
#include <random>

int main()
{
   std::random_device crypto_random_generator;
   std::uniform_int_distribution<int> int_distribution(0,9);
   
   int actual_distribution[10] = {0,0,0,0,0,0,0,0,0,0};
   
   for(int i = 0; i < 10000; i++) {
       int result = int_distribution(crypto_random_generator);
       actual_distribution[result]++;
   }

   for(int i = 0; i < 10; i++) {
       std::cout << actual_distribution[i] << " ";
   }
   
   return 0;
}

std::random_device is used in the same way as a pseudo random value generator is used.

However std::random_device may be implemented in terms of an implementation-defined pseudo-random number engine if a non-deterministic source (e.g. a hardware device) isn’t available to the implementation.

Detecting such implementations should be possible via the entropy member function (which return zero when the generator is completely deterministic), but many popular libraries (both GCC’s libstdc++ and LLVM’s libc++) always return zero, even when they’re using high-quality external randomness.