[boost] Bug in random/uniform_real.hpp

From: Michael Stevens (Michael.Stevens_at_[hidden])
Date: 2003-07-26 16:04:14

>
> Dear Boosters, dear Jens,
>
> After some debugging I realized another bug in the new version of the 
> Boost random number library. uniform_real has the following operator():
>
>   result_type operator()(Engine& eng) { return eng() * (_max - _min) + 
> _min; }
>
> This does not work if the underlying engine does not give random 
> numbers in an interval other than [0,1), and thus fails with all of 
> the integer based engines in the Boost random library. Is it necessary 
> to wrap the engine in a uniform_01 object? But then it would fail for 
> the uniform_integer distributions? Either I am missing something here 
> or the Boost random library is in an inconsistent and broken state. 
> Could Jens or somebody else help me?
>
> Matthias

Hi Matthias,

Before using the new Boost::random it is important to FORGET everything 
you new about the old library. I spent a day being confused. The 
interface really has changed!! Certainly the new interface has its 
advantages, but for oldtimers the changes are confusing.

The distributions all expect to be called with random numbers in the 
[0,1) range.  In general you should always marry up a Engine (integer or 
whaterever) with a Distribution using variate_generator. This results in 
a function object that can used in the same way as the old distributions.

The other big change is to the meaning of template parameters. The user 
now has the ability to control if the types use references to sub 
objects or values. Previously they always stored sub objects as 
references. This change requires some tricky template type handling in 
variate_generator. This has the side effect of breaking compatibilty 
with compilers lacking partial template specialisation when the template 
parameters are references.

GCC 2.95.3 also seems to be broken by the changes but I unsure at the 
moment of the root cause.

Hopefully Jens can update the boost documentation to match the new code.

One minor bug report of my own. variate_generator.hpp line 97 has an 
extra trailing ";".  See

  variate_generator(Engine e, Distribution d)
    : _eng(decorated_engine(e)), _dist(d) { };

I also have one question uniform_01 no longer fits, and is not part of 
the standard proposal. Will this be dropped from Boost to match?

All the best,
    Michael