Re: [boost] [range/iterator] operator[] forwarding

From: David Abrahams (dave_at_[hidden])
Date: 2006-10-04 11:45:58

Thorsten Ottosen <thorsten.ottosen_at_[hidden]> writes:

> Shunsuke Sogame wrote:
>> Hi gurus, under RC_1_34,
>> 
>>     #include <boost/range.hpp>
>>     #include <boost/iterator/counting_iterator.hpp>
>> 
>>     int main()
>>     {
>>         boost::make_iterator_range(
>>             boost::make_counting_iterator(0),
>>             boost::make_counting_iterator(5)
>>         )[2];
>>     }
>> 
>> 
>> VC++7.1 and GCC3.4 says a warning something like
>> "returning address of local variable or temporary".
>
> I think counting_iterator is not a valid random access iterator because 
> operator[]() does not yield a reference.

That is not a requirement of random access iterator.

>> counting_iterator is implemented by iterator_facade,
>> whose operator[] returns *value*.
>> In fact, the result type of iterator_facade's operator[] seems unspecified.
>> Is it possible to do forwarding of iterator's operator[], which is
>> tried by iterator_range?
>
> Counting iterator should define its reference-type to be the type it 
> returns from operator[]() (even though it is not a reference). Then I 
> think it would work.
>
> Do the authors of the iterator lib agree?

No.  There are good reasons that it works the way it does.  Please
read the iterator library docs and refer to the standard if you want
to understand this stuff; I don't have time to go into detail right
now, sorry.

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