[Boost][Random] Question on random_test.cpp

27 Mar 2009

      The following is an excerpt from the random_test.cpp file (part of the 
instantiate_urng function): 

 urng2 = urng; 

  { 
    // narrow stream first 
    std::ostringstream file; 
    file << urng; 
    // move forward 
    urng(); 
    // restore old state 
    std::istringstream input(file.str()); 
    input >> urng; 
    // std::cout << file.str() << std::endl; 
#if !defined(BOOST_MSVC) || BOOST_MSVC > 1300 // MSVC brokenness 
    // advance some more so that state is exercised 
    for(int i = 0; i < 10000; ++i) { 
      urng(); 
      urng2(); 
    } 
    BOOST_CHECK(urng == urng2); 
#endif // BOOST_MSVC 
  } 

My concern is that when a random number generator with an underlying double type (like fibonacci607) is stored to the stream and then pulled back out, then 
precision may be lost, and the underlying data is not guaranteed to be the same.  
If a double is written out to a stream in decimal form and then read back in the 
value may be changed.  Am I missing some sort of language guarantee that this is 
supposed to work?  I've tried this on AIX and the test failed at this point for 
fibonacci607 because some (but not all) of the underlying doubles had changed 
after the object was stored and retrieved. 

Thanks!

n1driver＠charter.net

Paul A. Bristow

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