Re: [Boost-users] Problem with iterator_facade

Subject: Re: [Boost-users] Problem with iterator_facade
From: Daniel Krügler (dsp_at_[hidden])
Date: 2008-09-17 07:21:00

Robert Dailey wrote:
> Hi,
> 
> I have the following iterator_facade class:
> 
> 	class PassIterator
> 		: public boost::iterator_facade<
> 			PassIterator,
> 			Pass,
> 			boost::forward_traversal_tag
> 		>
> 	{
> 	public:
> 		PassIterator();
> 
> 		PassIterator( Effect const& effect );
> 
> 	private:
> 		void increment();
> 		Pass& dereference() const;
> 
> 		Pass m_pass;
> 
> 		friend class boost::iterator_core_access;
> 	};
> 
> Note how my Pass object is stored by-value in my PassIterator class.
> When I attempt to compile this class, it fails because dereference()
> is const and it is trying to return a reference to a const member.
> However, if I make dereference() non-const, it still fails because the
> base class is calling into PassIterator::dereference() through a const
> member function as well, so dereference() *must* be const in order to
> be callable by boost.
> 
> How can I overcome this issue? const_cast? mutable? Thanks.

I guess you want:

class PassIterator
                : public boost::iterator_facade<
                        PassIterator,
                        const Pass, // #1
                        boost::forward_traversal_tag
		>

...
                const Pass& dereference() const; // #2
...
};

Alternatively you could do this:

class PassIterator
                : public boost::iterator_facade<
                        PassIterator,
                        Pass, // #1
                        boost::forward_traversal_tag,
                         Pass // #2
		>

...
                Pass dereference() const; // #3
...
};

Greetings from Bremen,

Daniel Krügler