[boost] uniform_real rather alarmingly non-random?

From: John Maddock (john_at_[hidden])
Date: 2006-03-17 14:28:30

I seriously hope I'm doing something wrong here, but the last 20 bits in a 
double returned from uniform_real appear to always be zero.  Is this by 
design?  I really hope not!

Here's the test code:

template <class T>
void test_random(T)
{
  boost::mt19937 rnd;
  boost::uniform_real<T> ur_a(0.5, 1);
  boost::variate_generator<std::tr1::mt19937, std::tr1::uniform_real<T> > 
gen(rnd, ur_a);
  for(int j = 0; j < INT_MAX-2; ++j)
  {
    T v = gen();
    // get the digits of T:
    unsigned long long i = (unsigned long long)std::floor(std::ldexp(v, 
std::numeric_limits<T>::digits));
    i &= 0xfffff;  // last 20 bits
    std::cout << std::hex << i << std::endl;
  }
}

Try calling with a double as an argument and it'll output nothing but 
zero's.

John.