From: Joel de Guzman (joel_at_[hidden])
Date: 2003-11-27 02:59:08
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);
}
Cheers,
-- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net