Re: [boost] math/statistics policy-interface feedback requested.

From: Stephan Tolksdorf (andorxor_at_[hidden])
Date: 2007-06-19 12:37:39

Hi John,

> The main disadvantage I've noticed at present, is that the mangled names of 
> the policy class - and therefore all the special functions etc - are *very* 
> long.  This has an impact on error messages: in particular we currently use 
> BOOST_CURRENT_FUNCTION to get a nice formatted name of a function that's 
> about to throw, but with function names a couple of pages long I don't think 
> that will be possible with this interface any more :-(
> 

What about moving the policy stuff into a traits class instead, combined 
with a policy id?

namespace boost { namespace math {

template <size_t ID>
struct policy_traits {
     // default policy
     static const bool check_domain = true;
     static const size_t internal_precision = 16;
     // ...
};

// predefined policies
const size_t default_policy_id = 0;
const size_t fast_policy_id = 1;
template <>
struct policy_traits<fast_policy_id>
        : public	policy_traits<default_policy_id>
{
     static const bool check_domain = false; // overwrite check_domain
};

// a simple size_t wrapper
template <size_t ID>
struct policy {
     static const size_t value = ID;
};

typedef policy<fast_policy_id> fast_policy;

} }

Users then could define their own policies by providing a new 
specialization of the policy_traits class for their (unique) ID and 
would call the special functions similar to

quantile(
     poisson(100),
     0.05,
     fast_policy
);

This would avoid the complicated meta-programming you suggested without 
compromising on the level of control.

Regards,
   Stephan