[boost] Re: std::sort(zip_iterator...) does not compile

From: David Abrahams (dave_at_[hidden])
Date: 2004-07-22 12:03:25

Anthony Williams <anthony_w.geo_at_[hidden]> writes:

> David Abrahams <dave_at_[hidden]> writes:
>
>> Anthony Williams <anthony_w.geo_at_[hidden]> writes:
>>
>>> Gunter Winkler <guwi17_at_[hidden]> writes:
>>>
>>>> I tried to use boost::zip_iterator to sort 2 parallel arrays. Unfortunatly
>>>> gcc does not compile the appended example because it can not convert
>>>>
>>>>    boost::detail::iterator_category_with_traversal<std::input_iterator_tag, 
>>>>    boost::random_access_traversal_tag>
>>>>
>>>> to either 
>>>>   std::bidirectional_iterator_tag
>>>> or
>>>>   std::random_access_iterator_tag.
>>>> Thus it cannot dispatch the correct stl::__copy_backward() procedure.
>>>>
>>>> Is sort() not yet supported? 
>>>
>>> As Dave already posted, zip_iterator doesn't support return forward or
>>> bidirectional iterators, so std::sort() cannot be used.
>>>
>>> What you need is something like my tuple iterator (tupleit.zip from the files
>>> area on the boost yahoo group), which *does* support use with std::sort ---
>>> the iterator category of the tuple iterator is the minimum category of the
>>> supplied iterators.
>>
>> How can sorting work?
>> What type is returned from its operator* ?
>
> For Forward, Bidi and Random-access Iterators, it returns a reference to a
> value held in the iterator.

I'm afraid those aren't conforming forward iterators :(

24.1.3:

    "If a and b are both dereferenceable, then a == b if and only if
    *a and *b are the same object. "

Which is why we have to change counting_iterator to not be a forward
iterator.

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