Re: Repost: [boost] Problems using iterator_adaptor with istreambuf_iterator

From: David Abrahams (dave_at_[hidden])
Date: 2003-03-26 12:09:17

hartmutkaiser_at_[hidden] (Hartmut Kaiser) writes:

> David Abrahams wrote:
>
>> >> I have a problem while using the iterator_adaptor templates
>> >> in conjunction with a istreambuf_iterator<char> (an 
>> >> input_iterator type). The problem shows up, because the 
>> >> istreambuf_iterator<char>::operator*()
>> >> implementation of the STL I'm using returns a value_type 
>> >> (char), but the dereference policy member expects to return 
>> >> it a reference.
>> >> 
>> >> It seems, that there should be a similar return type deduction for 
>> >> the
>> >> iterator_adaptor<>::operator*() function as already 
>> >> implemented for the
>> >> iterator_adaptor<>::operator->() function.
>> >
>> > Regards Hartmut
>> 
>> Which template are you having problems with?  Can you post 
>> some code which reproduces the problem?  Did you try 
>> explicitly specifying the iterator's reference type?
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Can't you solve this by passing char instead of char& as the 4th
template parameter?

> I've attached a minimal (braindead) sample, which reproduces the problem
> (it does not compile). 
>
> The input_iterator used as the Base iterator of the adaptor returns a
> value_type from its operator*(), but the iterator_adaptor template
> expects a reference here. Surely I could solve it by adding a member of
> type value_type to my policy class, using it as a buffer and the
> reference to it may be returned, but isn't it better to resolve it in
> the adaptor? Even more because a congruent problem, which happens often
> with the operator->() is already solved through a proxy class. I believe
> that the operator*() problem may be solved in a similar fashion.
>
> Regards Hartmut
>
>
> #include <iterator>
> #include <boost/iterator_adaptors.hpp>
>
> using namespace boost;
>
> struct dummy_input_iterator 
> {
>     typedef std::input_iterator_tag iterator_category;
> 	typedef char value_type;
> 	typedef size_t difference_type;
> 	typedef char * pointer;
> 	typedef char & reference;
> 	
> 	value_type operator*() const { return 42; }
> 	dummy_input_iterator & operator++() { return *this; }
> 	dummy_input_iterator operator++(int) { return *this; }
> };
>
> typedef iterator_adaptor<
>         dummy_input_iterator, default_iterator_policies,
>         char, char &, char *
>     > adapted_dummy_iterator;

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