Generate random 128 bit in c++ without boost

Question

I have a datatype of size 128 bit, and I like to generate a random value without using boost or any third party headers/libraries. I wrote the below code snippet and it's working fine, but I want to know if there are any issues/pitfalls with the approach.

#include <stdlib.h>
#include <time.h>
#include <array>
#include <iostream>
#include <random>
int main() {
    constexpr int size = 16;
    std::array<std::uint8_t, size> randomID;
    std::mt19937_64 gen_{std::random_device{}()};
    std::uniform_int_distribution<std::uint8_t> dis_{1};


    for (int i = 0; i < randomID.size(); i++) {
        randomID[i] = dis_(gen_);
        std::cout << unsigned(randomID[i]) << " ";
    }
    return 0;
}

Answer 1

One issue with the posted approach is that it will never generate a 0 octet.

std::uniform_int_distribution<std::uint8_t> dis_{1};
//                                               ^   the range will be [1, 255] 

You could also use a distribution of uint64_t and spread the bits in the array

std::uniform_int_distribution<std::uint64_t> dis_{};

for ( size_t i{}; i < randomID.size(); i += 8 )
{
    auto r{ dis_(gen_) };
    for ( unsigned j{}; j < 8; ++j )
    {
        randomID[i + j] = (r >> (j * 8)) & 0xFF;
        std::cout << unsigned(randomID[i + j]) << " ";
    }
}

Consider also using a std::seed_seq to initialize std::mt19937.

std::random_device rd{};
std::seed_seq ss{ rd(), rd(), rd() };
//                ^^^^^^^^^^^^^^^^ Increase the entropy.
std::mt19937_64 gen_{ ss };