id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
351,Diff in state of mersenne_twister gen between GCC3.41 & CW9,toth21,jmaurer,"{{{
I have found a bug with the mersenne_twister random 
number generator class, where results when calling for a 
random number vary between codewarrior 9.0 and gcc 
3.4.1 with boost 1.31.0.

A simple example is shown below:

#include <boost/random/mersenne_twister.hpp> 
//for mt11213b

typedef boost::mt11213b     base_generator_type;

class foo
{
 public:
  foo(const base_generator_type gen):m_gen(gen)
  {}

  ~foo(void) {}

  rand_type operator() ( void )
  {
   const float x = m_gen();

   return x;
  }

 private:

  base_generator_type m_gen;
}

int main(void)
{

 base_generator_type gen( (uint32_t) 2000);  
//seed is not important

 foo f(gen);

 cout << ""Random result is "" << f() << endl;
}

The resulting numbers will be different between 
codewarrior 9.0 running on windows (xp or 2000) and 
gcc 3.4.1 running on Suse linux 7.3.(Hope that is enough
info).

I tracked the cause of the bug down to class 
initialization. In the case of CW9.0, the compiler when 
constructing a foo, will use a mersenne_twister copy 
ctor when initializing m_gen, while in the case of
gcc 3.4.1 it will choose the following in the .hpp file

  template<class Generator>
  explicit mersenne_twister(Generator & gen) { seed
(gen); }

therefore using the provided generator as a seed to the 
new generator rather than just copying its state, even 
though no template parameter is provided.

Anyways I will leave it up to the more informed to decide 
which is more desirable/proper; however, this is a fairly 
nasty bug at the moment as you can not reproduce 
results between these two platforms when using this 
random number generator for your random numbers.


}}}",Bugs,closed,,random,None,,fixed,,