[boost] Re: Proposal for New Template Metafunction 'is_iterator'

From: David Abrahams (dave_at_[hidden])
Date: 2004-08-22 20:14:03

"Alex Chovanec" <achovane_at_[hidden]> writes:

> "David Abrahams" <dave_at_[hidden]> wrote in message
> news:ullg6287u.fsf_at_boost-consulting.com...
>> "Alex Chovanec" <achovane_at_[hidden]> writes:
>> >
>> > The 'is_iterator' struct template is a model of the "integral constant
>> > expression" concept. Thus, 'is_iterator<T>::value' evaluates to true
>> > if-and-only-if type T defines iterator traits
>>
>> I don't believe that's detectable without generating an error.  Don't
>> forget that iterator_traits can be specialized; there's no reason the
>> iterator has to contain members.  Quick test: what is
>> is_iterator<int*>::value?
>
> I don't really see why this a problem. 'is_iterator' can be specialized too,
> and that is what I have done for the case of pointers:
>
>     template <typename T>
>     struct is_iterator<T *>
>     {
>         typedef is_iterator type;
>         typedef bool value_type;
>
>         BOOST_STATIC_CONSTANT(value_type, value = true);
>     };
>
> So 'is_iterator<int *>::value' evaluates to true without generating a
> compile error, as it should.

*Any* iterator can be implemented the same way.  An iterator class
 does not have to have a value_type member!

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com