[boost] Re: iterator_facade operator[]

From: Alberto Barbati (abarbati_at_[hidden])
Date: 2004-03-08 09:17:02

David Abrahams wrote:

> Alberto Barbati <abarbati_at_[hidden]> writes:
> 
>>Problem is that there is something wrong with the implementation of
>>operator[] in iterator_facade and the regression test does not compile
>>anymore :-( 
> 
> What makes you say that there's something wrong with iterator_facade's
> operator[]?  Is it possible that the regression test makes invalid
> assumptions about the way an iterator's operator[] is supposed to work?

The problem is triggered by the expression

     BOOST_CHECK(it[0] == 2);

found in file base_test.cpp line 157. VC .NET 2003 gives the following 
error message:

error C2678: binary '==' : no operator found which takes a left-hand 
operand of type 
'boost::detail::operator_brackets_result<Iterator,Value,Reference>::type' 
(or there is no acceptable conversion)
with
[
Iterator=boost::cb_details::cb_iterator<boost::circular_buffer<Integer>,Integer>,
Value=Integer,
Reference=Integer &
]

"it" is defined as circular_buffer<Integer>::iterator. As the right hand 
side is not an "Integer" I also tried writing:

     BOOST_CHECK(it[0] == Integer(2));

but the error message stays the same.

>>However, if I hack iterator_facade::operator[] to avoid
>>the use of the operator_brackets_proxy class
> 
> That's unneccessary.  Any iterator_facade behavior you want to replace
> can simply be added to your derived iterator class.

Yes. Sure you are right. I feel kind of stupid, I should have thought 
about it in the first place.

I am uploading a revised version of circular_buffer_new_iterators_v2.zip 
that overrides operator[]. All regression tests passes with VC7.1. I 
also made the implementation members private and fixed the comments.

Anyway you'll agree that the issue is quite general and it might be 
interesting to investigate if we could find a solution at the 
iterator_facade level.

>>all regression tests pass. Maybe it would be good to discuss this
>>problem of the iterator_facade in a different thread.
> 
> 
> Here we are!

:-)

Alberto