Re: [boost] Re: safe_bool alternative?

From: Daryle Walker (darylew_at_[hidden])
Date: 2003-11-28 21:07:06

On 11/27/03 2:59 AM, "Joel de Guzman" <joel_at_[hidden]> wrote:

> Joel de Guzman <joel_at_[hidden]> wrote:
>> Daryle Walker <darylew_at_[hidden]> wrote:
>>> On 11/24/03 10:47 PM, "Joel de Guzman" <joel_at_[hidden]> wrote:
>> 
>>> BTW, Spirit does have a solution for this problem, right?  Otherwise, you
>>> have a major bug!
>>> 
>>> (Note that no matter how you define Boolean-conversion, you need to
>>> explicitly provide operators == and != for your type, "poisoned" if such
>>> objects aren't supposed to be comparable, and conventional if they are.)
>> 
>> You must've missed this:
>> 
>> Second take:
>> 
>>     template <typename T>
>>     struct safe_bool
>>     {
>>         int* dummy;
>>         typedef int* safe_bool::*type;
>>     };
>> 
>>     template <typename T>
>>     inline typename safe_bool<T>::type
>>     make_safe_bool(bool cond)
>>     {
>>         return cond ? &safe_bool<T>::dummy : 0;
>>     }
>> 
>> Can you spot some problems with this too?
> 
> Some tweaks:
> 
>   template <typename T>
>   struct safe_bool
>   {
>       T* dummy;
>       typedef T* safe_bool::*type;
>   };
> 
>   template <typename T>
>   inline typename safe_bool<T>::type
>   make_safe_bool(bool cond)
>   {
>       return cond ? &safe_bool<T>::dummy : 0;
>   }
> 
> Here's a sample:
> 
>       operator safe_bool<match<nil_t> >::type() const
>       {
>           return make_safe_bool<match<nil_t> >(len >= 0);
>       }

I'm assuming that this operator is in the class definition of
"match<nil_t>", right?  And that all your other classes use "safe_bool", but
substitute themselves for the "T" parameter here, right?  If you do that,
then you shouldn't have to worry about objects of different Spirit types
comparing with each other.  (As I said in my note above, objects of the same
Spirit type still indirectly get operators == and != because they have an
one-step conversion to the same built-in singular type.  If you haven't
already, you need to add those operators to your Spirit types, and either
define them properly or "poison" them private.)

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT hotmail DOT com