Надо сгенерировать 8-битное значение

unsigned long long element = (((unsigned)rand() << 32) | (unsigned)rand());

// uniform_real_distribution
#include <iostream>
#include <random>
 
int main()
{
  const int nrolls=10000;  // number of experiments
  const int nstars=95;     // maximum number of stars to distribute
  const int nintervals=10; // number of intervals
 
  std::default_random_engine generator;
  std::uniform_real_distribution<double> distribution(0.0,1.0);
 
  int p[nintervals]={};
 
  for (int i=0; i<nrolls; ++i) {
    double number = distribution(generator);
    ++p[int(nintervals*number)];
  }
 
  std::cout << "uniform_real_distribution (0.0,1.0):" << std::endl;
  std::cout << std::fixed; std::cout.precision(1);
 
  for (int i=0; i<nintervals; ++i) {
    std::cout << float(i)/nintervals << "-" << float(i+1)/nintervals << ": ";
    std::cout << std::string(p[i]*nstars/nrolls,'*') << std::endl;
  }
 
  return 0;
}

unsigned long long element = (((unsigned long long)rand() << 32) | (unsigned)rand());

#include <climits>
#include <random>
#include <chrono>
 
  std::default_random_engine g(std::chrono::system_clock::now().time_since_epoch().count());
  std::uniform_int_distribution<unsigned long long> distribution(0, ULLONG_MAX);
 
unsigned long long number = distribution(g);