Re: [boost] Report from Berlin C++ Standards Committee meeting

From: David Abrahams (dave_at_[hidden])
Date: 2006-04-16 17:56:39

Howard Hinnant <howard.hinnant_at_[hidden]> writes:

> Actually in this particular case, all we need is a more smartly  
> designed std::iterator_traits class:
>
> Spec:
> ---
> template <class Iterator>
> struct iterator_traits
> {
> };
>
> However if Iterator has the nested types: difference_type,  
> value_type, pointer, reference and iterator_category, and if  
> Iterator::iterator_category (if it exists) is implicitly convertible  
> to either std::input_iterator_tag, or std::output_iterator_tag, then  
> the struct iterator_traits is instead:
>
> template <class Iterator>
> struct iterator_traits
> {
>      typedef typename Iterator::difference_type   difference_type;
>      typedef typename Iterator::value_type        value_type;
>      typedef typename Iterator::pointer           pointer;
>      typedef typename Iterator::reference         reference;
>      typedef typename Iterator::iterator_category iterator_category;
> };
>
> Plus the pointer specializations...
> Plus any user-defined specializations...
> ---
>
> Such an iterator_traits is easy to implement today with has_member_*  
> and is_convertible.  And now you can say iterator_traits<any type at  
> all>.  The instantiation always compiles and may or may not have the  
> nested typedefs.
>
> Then you can make an is_input_iterator trait:
>
> A type T is an input iterator if iterator_traits<T> has a member  
> called iterator_category that is convertible to std::input_iterator_tag.

...which will fail even for some types that are legit members of std::
in C++98, like

   std::iterator<std::input_iterator_tag, int>

(false positive), not to mention types that inherit privately from
types like the one above, or just have those names as private members
(compile-time error).

I don't mean to rain on this parade every time you bring up the idea,
Howard; it's a nice hack.  I just don't want anyone to be confused
about that: it's a nice hack that works most of the time... and
sometimes breaks.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com