Re: [boost] [constrained_value] New version

Subject: Re: [boost] [constrained_value] New version
From: Robert Kawulak (kawulak_at_[hidden])
Date: 2008-11-18 20:02:06

Hi Neal,

> From: Neal Becker
> I thought to use 'unconstrained', but this isn't really what 
> I need.  I have a fixed_pt integer class, 
> 
> //! signed fixed pt
> template<int int_bits, int frac_bits, typename base_type=int, 
> typename error_policy=cv::throw_exception<>, typename 
> round_policy=rnd<base_type> >
> struct fixed_pt  :
> ...
>   typedef typename cv::bounded_int<base_type, min, 
> max,error_policy>::type val_t;
> 
> When I want to turn off checking, it seems the thing to do is 
> use an error_policy which ignores the error.  I've been using 
> this, which I got from you some time ago:
> 
>     struct no_error_policy { 
>       template <typename V, class C> 
>       void operator () (V & old_val, const V & new_val, C &) 
>       { 
> 	old_val = new_val; 
>       } 
>     };
> 
>  
> So I can write:
>   fixed_pt<16,5,int,cv::no_error_policy> f;
> 
> Maybe this is a useful addition to the lib, or at least as an example?

OK, now I think I see what you want to do.

I don't think using this policy is a good idea for two reasons:
- as I've written before, it is not a correct error policy in a general case. It
will break the bounded object's invariant (and fire an assertion) if you assign
an invalid value;
- it will make it harder for the compiler to optimize the unnecessary checks
away.

What I would suggest is using bounded_int when there is actually a need for
error checking, and using unconstrained when there should be no error-checking:

        struct no_error_checking {}; // a tag to indicate no checks at all

        template<... typename base_type = int, typename error_policy =
cv::throw_exception<> ...>
        struct fixed_pt {

                typedef typename if_<
                        is_same<error_policy, no_error_checking >::type, 
                        cv::unconstrained<base_type>::type,
                        cv::bounded_int<base_type, min, max, error_policy>::type
		>::type val_t;

Or, if there are no issues with the interface compatibility, you may use the
underlying value type directly:

                typedef typename if_<
                        is_same<error_policy, no_error_checking >::type, 
                        base_type,
                        cv::bounded_int<base_type, min, max, error_policy>::type
		>::type val_t;

Hope this helps,
Robert