[boost] New iterator adaptors questions

[Vladimir Prus]

I'm still having problems with using the new iterator adaptors, when trying to create 
input iterator with the help of 'iterator_facade'. The complete code 
is attached. The problems are with 'deference' method.

1. iterator_facade insists on having const 'deference', while it seems that input 
iterators frequently
alter internal state during dereferencing. What is the rationale for the current 
behaviour.

2. Unless I explicitly specify reference type, iterator_facade will try to use 
ValueType& as reference type,
which is wrong in my case, and, I'd think, for all input iterators. Again, what is the 
rationale?

TIA,
Volodya
#include <boost/iterator.hpp>
#include <boost/iterator/iterator_adaptor.hpp>

#include <iostream>
#include <sstream>

class adapted_iterator :
    public boost::iterator_facade< adapted_iterator, std::string,
                                   std::input_iterator_tag>
{
    typedef boost::iterator_facade< adapted_iterator, std::string,
                                    std::input_iterator_tag> base;
public:
    adapted_iterator() : is(0) {}
    adapted_iterator(std::istream* is) : is(is) {}

private:
    friend class boost::iterator_core_access;

    base::reference 
    //std::string 
    dereference() const
    {
        std::string s;
        getline(*is, s);
        return s;
    }

    void increment()
    {
    }
    
    bool equal(const adapted_iterator& another) const
    {
        bool at_eof1 = !is || is->eof();
        bool at_eof2 = !another.is || another.is->eof();
        return (at_eof1 == at_eof2);
    }

private:
    std::istream* is;
    
};

int main()
{
    std::stringstream ss("1  \n234\n\n45");
    
    for(adapted_iterator i(&ss), e; i != e; ++i) 
    {
        std::cout << "'" << *i << "'\n";
    }
    return 0;
}

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